The Prophet (peace be upon him) met some of the Yathribites belonging to the Khazraj at Aqabah (20) when he went to preach Islam to the tribes throughout the tenure of pilgrimage. He told them about Islam and called on them to serve God alone, reciting some Quranic verses in the process. As these people lived in Yathrib side by side with the Jews who often told them that an Prophet (peace be upon him) of God was soon to come, they said to one another: By God, this is the same thing that the Jews informed us; Lo, nobody should now get ahead of you. Thereupon they accepted his teachings and embraced Islam. They also said to the Prophet, When we left our people, conflict and hatred divided them more than any other. Perhaps God will unite them through you. We shall inform them to accept this religion of yours which has been accepted by us, and if God unites them on you, then no man shall be honored more than you. (Ibn Hisham, Vol. I, pp. 428-29)
These men returned to their homes after accepting Islam, where they told others about the Prophet (peace be upon him) and invited them to accept the new faith. Islam quickly spread in Madinah until there was no home left of the Ansaar wherein the Prophet (peace be upon him) was not mentioned. (Ibn Hisham, Vol. I, pp. 428-29)
FIRST PLEDGE OF AQABAH
At the pilgrimage the next year, twelve men belonging to the Ansaars met the Prophet (peace be upon him) at Aqabah. They pledged themselves to the Prophet (peace be upon him) vowing neither to commit theft nor fornication, nor to kill their children, to obey him in what was right, and to associate nothing with God. When these people left Madinah, the Prophet (peace be upon him) sent Musab Ibn Umayr (May Allah be pleased with them) with them to teach the Quran to the people there as well as to expound Islam and instruct them about the religion; wherefore Umayr came to be called The reader in Madina. He lived with Asad Ibn Zurara and also led prayers. (Ibn Hisham Vol. I, p. 434)
THE REASON FOR ANSAARS ACCEPTANCE OF ISLAM
It was a critical juncture when God afforded the opportunity of helping and defending Islam to the Aus and the Khazraj, (21), the two influential tribes of Yathrib. For there was nothing more precious at the moment than to own and accept Islam, they were really fortunate in getting the most relevant and timely opportunity to take precedence over all other tribes of Hijaz in welcoming and defending the religion of God. They overshadowed their compatriots since all the tribes of Arabia, in general, and the Quraysh, in particular, had proven themselves ungrateful as well as incompetent to take advantage of the greatest favor bestowed upon them.
And Allah guides whom He wills to a straight path
Diverse causes and circumstances, proceeding from the will of Almighty God, had opened the door for the acceptance of Islam by the Aus and the Khazraj. These tribes were not of the Meccan Qurayshites type for unlike them, the Aus and the Khazraj were kind-hearted and sweet-tempered, immune from the Qurayshite traits of immoderation, stubbornness and vanity, and hence they were responsive, open to reason. These were the characteristics inherited from their progenitors, the Yemenites, about whom the Prophet (peace be upon him) had remarked after meeting one of their deputations: The people of Yemen have come to you, and they have the tenderest hearts. Both these tribes of Yathrib originally belonged to Yemen for their forefathers had come down from there. Commending the merits of these people, God has said in the Quran:
Those who entered the city and the faith before them love those who flee unto them for refuge, and find in their breast no need for that which hath been given them, but prefer the fugitives above themselves though poverty becomes their lot.
Another reason was that continuous internecine collision had already exhausted both tribes. Enervated and distracted by the famous battle of Buath fought a short time ago (about 615 AD), the said tribes were desirous of peace and harmony and wanted to avoid renewal of warfare. Such was their anxiety for peace that the first Muslims of Madinah had said to the Prophet (peace be upon him), When we left our people, discord and conflict and enmity divided them more than any other. Perhaps God will unite them through you, and if God unites them on you, then no other man will be more than honored as you do. Aisha (May Allah be pleased with her) once said that the battle of Buath was really a divine intervention and a blessing in disguise which served as a prelude to the Prophets (peace be upon him) migration to Madinah.
Yet another reason was that the Quraysh, like the rest of the Arabian tribes, had for a long time lost touch with prophethood and the prophets and had hardly any recollection of their teachings. Deeply immersed in ignorance and idolatry as well as being completely strangers to the arts of reading and writing, they had become overzealous heathens; actually, they had but little contacts even with the Jews and Christians, the followers of the prophets and their scriptures (although these had since been distorted). This was plain, plum fact to which the Quran makes a reference in these words:
In order that you may warn a people whose forefathers were not warned, so they are heedless
But the Aus and the Khazraj were neighbours of Yathrib Jews whom they heard talking about the prophets and reciting their scriptures. The Jews often warned them that a prophet was to come in the later times with whom they would ally themselves and kill the heathens just as the people of Ad and Iram were massacred. (Tafsir Ibn Kathir, Vol. I, p. 217)
And when there cometh unto them a Scripture from Allah, confirming that in their possession - though before disbelieved - and when there cometh unto them that which they know (to be the Truth) they disbelieve therein. The curse of Allah is on disbelievers.
Aus and Khazraj as well as other Arab tribes settled in Madinah were heathens like the idolatrous Quraysh and the rest of the Arabs. But unlike them, they had become accustomed to the idea of revelation in the form of a scripture of supernatural origin, prophecy, prophetship, inspiration, requital and the hereafter. This was courtesy of their uninterrupted association with the Jews of the city from whom they had business transactions, made war and peace and lived side by side. They had, thus, come to know the teachings of the prophets of old and the reason why God sends them from time to time. This was of great advantage to these people, for, when they learned about the Prophet (peace be upon him) on the occasion of Hajj at Mecca, they at once grabbed the opportunity as if they were already prepared for it.