From Sahih Muslim (Hadith 510):
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 were not for me, he would have been in the bottom-most depth of Hell
Reference: Muslim 510