Shaykh Rabee’ Ibn Haadee al-Madkhalee
The noblest of all the Prophets and the last of them, Muhammad ibn 'Abdullaah (sal-Allahu `alayhe wa sallam). The one sent with the greatest, most complete and most comprehensive message. The one whom Allah sent as a mercy for the worlds, and as a bringer of good tidings and a warner, and as a Caller to Allah with His permission, and a shining light. He did not leave anything good except that he guided his Ummah to it and pointed it out to them, nor anything evil except that he warned them against it. So which of the fundamentals of Islaam did this exalted Prophet begin with? What was the starting point for his da'wah? He (sal-Allahu `alayhe wa sallam) began with what all the Prophets began with, and started where they started in their da'wah: calling to the 'aqeedah of tawheed, and calling for all worship to be made purely and sincerely for Allah alone. He began with the testification, "None has the right to be worshipped except Allah, Muhammad is the Messenger of Allah," (Laa ilaaha illAllah Muhammadur Rasoolullaah). Can it be imagined that he or any of the other Prophets would begin with anything but this great principle, the foundation of all the divine Messages?! Allah's Messenger (sal-Allahu `alayhe wa sallam) began with this fundamental principle, so the first thing which the people heard was, 'Witness that none has the right to be worshipped except Allah.' So the proud and haughty said.
"The unbelievers said, 'Has he made all that is to be worshipped a single God, who alone is to be called upon and hears all invocations. This is a very curious thing which we have not heard of.' So the leaders amongst them went off saying to one another, 'Continue in what you are upon and persist in worshipping your idols. He only says this to gain ascendancy over us.'" [Soorah Saad (38):5-6]
He continued calling to this lofty principle and highest goal throughout the Meccan period of his Messengership, for thirteen years. He did not become weary or languid. He patiently bore all types of harm to propagate this principle, since no religious duties or pillars of Islaam were obligated upon him, except for the Prayer which was obligated in the tenth year of Prophethood, and apart from the excellent manners of keeping ties of relationship, truthfulness and chastity which he commanded his people with. However the core of his da'wah and the cause of dispute and opposition to him was that great fundamental principle. So Allah charged this noble Prophet (sal-Allahu `alayhe wa sallam) with the particular duty of establishing this great principle. Allah, the Most High, says:
"Indeed We have sent down the Book to you, O Muhammad, with the truth, so worship Allah alone making all religion purely and sincerely for Him. Indeed the religion that is free of all taint of shirk is alone what is acceptable to Allah. As for those who take and invoke helpers and protectors besides Allah, then they say, 'We only worship them so that they should intercede for us and bring us nearer to Allah.' Indeed Allah will establish judgement between them regarding the matters of religion about which they dispute." [Soorah az-Zumar (39):2-3]
Allah, the Most High, says:
"Say, O Muhammad (sal-Allahu `alayhe wa sallam), 'Allah has commanded that I should worship Him alone, purely and sincerely, not associating anything with Him, and I am commanded to be the first of this nation who submits to Him as a Muslim, making all worship purely for Him.' Say, O Muhammad, 'I fear should I disobey my Lord in this, the torment of a great Day.' Say, 'It is Allah alone that I worship, making all of my worship purely and sincerely for Him, and not worshipping anything else besides Him.'" [Soorah az-Zumar (39):11-14]
"Say, O Muhammad (sal-Allahu `alayhe wa sallam), 'Indeed my Prayer, my sacrifice, my living and my dying are all purely and solely for Allah, Lord of all the Worlds. There is no share of any of that for other than Him. That is what My Lord ordered me, and I am the first of this nation to submit to Allah as a Muslim.'" [Soorah al-An'aam (6):162-163]
He also commanded him to call all of the people to fulfil and implement this principle, and to proceed upon it. Allah, the Most High, says:
"O Mankind, single out your Lord with worship; He who created you and all those who came before you so that you may be of those who seek to avoid Allah's punishment and anger; those whom Allah is pleased with. He Who has made the earth a resting place for you and has made the sky a canopy, and sent down rain from the sky, and brought out with it crops and fruits from the earth as provision for you. So do not set up rivals with Allah in your worship whilst you know that you have no Lord besides Him." [Soorah al-Baqarah (2):21-22]
Allah, the Most High, says:
"The God Who alone has the right to be worshipped is a Single God. So do not worship anything besides Him, nor associate anything in worship with Him, the Most Merciful, the Bestower of Mercy." [Soorah al-Baqarah (2):163]
Allah, the Most High, says:
"Say, O Muhammad (sal-Allahu `alayhe wa sallam), to all of the people, 'I am the Messenger of Allah to you all. Allah, to Whom belongs the dominion of the heavens and the earth and everything therein. None has the right to be worshipped but Him. He Who alone gives life and death. So affirm true faith in Allah and in His sole right to worship, and believe in His Messenger, the Unlettered Prophet (Muhammad sal-Allahu `alayhe wa sallam) who believes in Allah and His Words, and follow him, so that you may be rightly guided.'" [Soorah al-A'raaf (7):158]
The Aayaat in this regard are very many, and what we have quoted is an example showing the methodology of Allah's Messenger (sal-Allahu `alayhe wa sallam) in calling to tawheed.
Then with regard to the Sunnah, there are many things clearly showing that Allah's Messenger (sal-Allahu `alayhe wa sallam) began his da'wah with tawheed and completed it with it, and continued calling to it from the beginning to the end, throughout his (sal-Allahu `alayhe wa sallam) life.
'Amr ibn 'Abasah as-Sulamee, radiyAllahu 'anhu, who said,
"During the times of Ignorance I used to hold that the people were upon error and futility, and they used to worship the idols, then I heard of a man in Makkah who received revelation. So I set out upon my riding beast and came to him. At that time he was in hiding due to oppression of his people. So I behaved in a way that enabled me to gain access to him in Makkah, which I did. So I said to him, 'What are you?' He replied, 'I am a Prophet.' So I said, 'And what is a Prophet?' He said, 'Allah has sent me as a Messenger.' So I said, 'And what is it that He has sent you with?' He said, 'I have been sent to order the joining of ties of relationship, to break the idols, so that Allah is worshipped alone and nothing at all is associated in worship along with Him.' So I said, 'And who is with you upon this?' He said, 'A free man and a slave.' He said, 'And at that time Aboo Bakr and Bilaal were with him from those who believed in him...'" [Reported by: Muslim (Eng. trans. 2/395 no.1812) and Ahmad in al-Musnad (4/112)]
When 'Amr ibn al-'Aas and 'Abdullaah ibn Rabee'ah al-Makhzoomee went to an-Najaashee, the ruler of Abyssinia, to speak to him in order to incite him against the Muslims who had emigrated to Abyssinia, they said,
'O king, some of our foolish youths have abandoned their religion and come to your land. They have split away from their own people and have not entered your religion. Rather they have come up with an innovated religion which is not known either to us or to you...'
So the Najaashee asked the Muslims,
'What is this religion which you have split from your people and not entered my religion or the religion of any of these nations?!'
So Ja'far ibn Abee Taalib was the one who replied, and he said to him,
'O king, we were a people from the people of ignorance who worshipped idols, and we ate unslaughtered meat and committed foul acts, and we cut off ties of relationship, treated our neighbours in an evil manner and the strong amongst us used to devour the weak. So we were upon that until Allah sent a Messenger to us from amongst us. We knew his lineage, his truthfulness, his trustworthiness and his chastity. So he called us to Allah, that we should single Him out and worship Him alone and renounce everything which we and our fathers used to worship besides Him, all stones and idols; he commanded us to speak the truth and to fulfil trusts; to join ties of relationship; to live in a good manner with our neighbours; to avoid forbidden acts and shedding blood; and he forbade us from foul acts and from falsehood; and he forbade us from unlawfully devouring the wealth of orphans and falsely accusing chaste women. He ordered us to worship Allah alone and not to worship anything along with Him...' He said, 'So he told him the affairs of Islaam,' 'So we attested to his truthfulness and believed him and followed him in that which he came with. So we worshipped Allah alone, and did not worship anything else besides Him, and we made forbidden that which He forbade us, and declared lawful that which He allowed for us. So our people became enemies to us and tortured us and tormented us to turn back from our religion to the worship of idols, and so that we should allow the filthy actions which we used to allow. So when they oppressed us, treated us cruelly and made it unbearable for us, and came between us and our religion, then we left and came to your land, and we preferred you to others and wished to live in your company, and we hoped that we would not be oppressed while with you...'" 
Also amongst the questions which Hirqal (Heraclius) asked Aboo Sufyaan, at the time of the peace of Hudaybiyyah, about Allah's Messenger (sal-Allahu `alayhe wa sallam) was that he asked Aboo Sufyaan,
"What does he command you?"
So Aboo Sufyaan replied,
"He says, 'Worship Allah alone and do not worship anything else along with Him, and abandon what your fathers say. And he orders Prayer, charity, chastity and the joining of ties of relationship.'" [Reported by: al-Bukhaaree (Eng. trans. 1/7/no.6) and it is a long hadeeth]
So these ahaadeeth clearly show us what the da'wah of Allah's Messenger (sal-Allahu `alayhe wa sallam) was in both the Makkan and Madinan periods.
 Reported by: Imaam Ahmad in his Musnad (1/202) and (5/290). Ahmad said,
"Ya'qoob (meaning Ibraaheem ibn Sa'd az-Zuhree) narrated to us (reliable): My father narrated to us, from Muhammad ibn Ishaaq: Muhammad ibn Muslim ibn 'Ubaydillaah ibn Shihaab narrated to me: from Aboo Bakr ibn 'Abdir-Rahmaan ibn al-Haarith ibn Hishaam al-Makhzoomee: from Umm Salamah bint Abee Umayyah (meaning the Mother of the Believers, radiyAllahu 'anhaa)."
It is a chain of narration that is fully saheeh, except for Muhammad ibn Ishaaq, but he clearly states that he heard it directly and so his narration is hasan (good and acceptable).
Source: Manhaj ul-Ambiyaa fee ad-Da’wah illAllah (The Methodology of the Prophet's In Calling To Allah)