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 (ﷺ) 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 (ﷺ) 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).
Sahih Muslim Book 51, Hadith 4