Revelational Circumstances of the Qur'an: Surah 39 (Az-Zumar)

Say: O My slaves who have been prodigal to their own hurt! Despair not of the mercy of Allah, Who forgiveth all sins. Lo! He is the Forgiving, the Merciful.

Narrated Ibn Abbas
Some pagans who committed murders in great number and committed illegal sexual intercourse excessively, came to Muhammad and said, "O Muhammad! Whatever you say and invite people to, is good: but we wish if you could inform us whether we can make an expiration for our (past evil) deeds." So the Divine Verses came: 'Those who invoke not with Allah any other god, not kill such life as Allah has forbidden except for just cause, nor commit illegal sexual intercourse.' (25.68) And there was also revealed:-- 'Say: O My slaves who have transgressed against their souls! Despair not of the Mercy of Allah.' (39.53)