Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said that some people from among the polytheists had killed much and committed adultery much. Then they came to Muhammad (peace and blessings be upon him) and said: Tell us the expiation for the deeds we have done. So this verse was revealed: "And those who do not call upon any other deity along with Allah, and do not kill the soul which Allah has forbidden except by right, and do not commit adultery." [Surah al-Furqan: 68]
And this verse was revealed: "Say: O My servants who have transgressed against themselves, do not despair of the mercy of Allah." [Surah al-Zumar: 53]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18189