Sahih Muslim Book 1, Hadith Number 408.

Chapter : Intercession of the Messenger of Allah (may peace be upon him) for Abu Talib and some remission for him on this account.

It is reported on the authority of ‘Abbas b. Abd al-Muttalib that he said: Messenger of Allah, have you benefited Abu Talib in any way for he defended you and was fervent in your defence? The Messenger of Allah (may peace he upon him) said: Yes; he would be in the most shallow part of the Fire: and but for me he would have been in the lowest part of Hell.