It appeared simply impossible that the Prophet (peace upon him) would forgive Abu Sufyan. Time and again, he had tried his utmost to crush Islam, and many Muslims were killed at his hands. But the Prophet’s (peace upon him) merciful nature was extended to friend and foe alike. Abu Sufyan was forgiven and after twenty long years his heart was opened to the truth. He embraced Islam and declared his belief without reservation. The Prophet (peace upon him) then told Abu Sufyan to return to Makkah and inform the Quraysh that the Muslims would enter the city the following morning. Before his departure, al-‘Abbas made a request to the Prophet (peace upon him) that as Abu Sufyan was a proud man, it would be good to give him an honourable position. The Prophet took his uncle’s advice and said to Abu Sufyan, “Tell your people that he who enters the house of Abu Sufyan is safe, he who enters his own house is safe and he who enters the area of the Ka’bah is safe.”
Such was the merciful nature of the Prophet (peace upon him) that he bore no grudges against the idolaters of Makkah and was willing not only to forgive, but also to honour his former enemies.
Source: Al-Banna, S.M. Hasan, The Sirah of the Final Prophet, Awakening Publications, Swansea, UK, 2002. p.152-153.