Sahih Muslim Book 38, Hadith Number 6680.

Ibn ‘Umar reported that when ‘Abdullah b. Ubayy b. Salul died. His son ‘Abdullah b. ‘Abdullah (b.Ubayy) came to Allah’s Messenger (may peace be upon him) and begged him that he should give him his shirt which he would use as a coffin for his father, he gave him that. He then begged that he should conduct funeral prayer for him. Allah’s Messenger (may peace be upon him) had hardly got up to observe the prayer for him that ‘Umar stood up and caught hold of the garment of Allah’s Messenger (may peace be upon him) and said: Allah’s Messenger, are you going to conduct prayer for this man, whereas Allah has forbidden you to offer prayer for him? Thereupon Allah’s Messenger (may peace be upon him) said: Allah has given me an option as He has said: “You may beg pardon for them or you may not beg pardon for them, and even if you beg pardon for them, seventy times” (ix. 80), and I am going to make an addition to the seventy. He was a hypocrite and Allah’s Messenger (may peace be upon him) offered prayer for him and Allah, the Exalted and Glorious, revealed this verse: “Do not offer prayer for any one of them at all and do not stand upon their graves for (offering prayer over them)” (ix. 84).