Musnad Ahmad 2:14

Ya'qub narrated to us, my father narrated to us, from Ibn Ishaq, al-Zuhri narrated to me, from Ubayd Allah ibn Abd Allah ibn Utbah ibn Mas'ud, from Abd Allah ibn Abbas, who said: I heard Umar ibn al-Khattab saying: When Abd Allah ibn Ubayy died, the Messenger of Allah (peace and blessings of Allah be upon him) was called to pray over him. He stood up to go to him, and when he stood over him intending to pray, I moved until I stood at his chest and said: "O Messenger of Allah, [are you praying] over the enemy of Allah, Abd Allah ibn Ubayy, who said on such-and-such a day—" and he began counting his days. He said: The Messenger of Allah (peace and blessings of Allah be upon him) was smiling. When I persisted against him, he said: "Move away from me, O Umar. I have been given a choice, and I have chosen. It has been said: 'Ask forgiveness for them, or do not ask forgiveness for them; if you ask forgiveness for them seventy times, never will Allah forgive them' [At-Tawbah: 80]. If I knew that if I added to the seventy he would be forgiven, I would have added to it." He said: Then he prayed over him, walked with him, and stood at his grave until he finished with it. He said: He [Umar] marveled at my boldness toward the Messenger of Allah (peace and blessings of Allah be upon him), and Allah and His Messenger know best. He said: By Allah, it was only a short while later that these two verses were revealed: "And do not pray [the funeral prayer, O Muhammad,] over any of them who has died—ever—or stand at his grave. Indeed, they disbelieved in Allah and His Messenger and died while they were defiantly disobedient" [At-Tawbah: 84]. The Messenger of Allah (peace and blessings of Allah be upon him) never prayed over a hypocrite after that, nor did he stand at his grave until Allah, the Mighty and Majestic, took his soul.