Musnad Ahmad 1080:2

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from al-Zuhri, from Sa'id ibn al-Musayyib, from his father, who said: When death approached Abu Talib, the Prophet, peace and blessings of Allah be upon him, entered upon him while Abu Jahl and 'Abd Allah ibn Abi Umayyah were with him. He said: "O my uncle, say: 'There is no god but Allah,' a word by which I may argue for you before Allah." Abu Jahl and 'Abd Allah ibn Abi Umayyah said: "O Abu Talib, do you turn away from the religion of 'Abd al-Muttalib?" They continued speaking to him until the last thing he said to them was: "Upon the religion of 'Abd al-Muttalib." The Prophet, peace and blessings of Allah be upon him, said: "I will surely seek forgiveness for you as long as I am not forbidden from doing so." Then it was revealed: "It is not for the Prophet and those who have believed to ask forgiveness for the polytheists, even if they were relatives, after it has become clear to them that they are companions of Hellfire" [al-Tawbah: 113]. He said: And concerning him was revealed: "Indeed, you do not guide whom you love" [al-Qasas: 56].