Sahih Muslim Book 001, Hadith Number 0221.
Chapter : Islam effaces all the previous misdeeds and so do Migration and Pilgrimage.
It is narrated on the authority of Ibn 'Abbas that some persons amongst the polytheist had committed a large number of murders and had excessively indulged in fornication. Then they came to Muhammad (may peace be upon him) and said: Whatever you assert and whatever you call to is indeed good. But if you inform us that there is atonement of our past deeds (then we would embrace Islam). Then it was revealed:
And those who call not unto another god along with Allah and slay not any soul which Allah has forbidden except in the cause of justice, nor commit fornication; and he who does this shall meet the requital of sin. Multiplied for him shall be the torment on the Day of Resurrection, and he shall therein abide disgraced, except him who repents a believes and does good deeds. Then these! for the Allah shall change their vices into virtues. Verily Allah is Ever Forgiving, Merciful (xxv. 68-70).
Say thou: O my bondsmen woo have committed extravagance against themselves despair not of the Mercy of Allah I Verily Allah will forgive the sins altogether. He is indeed the Forgiving, the Merciful (xxxix. 53).