Narrated `Abdullah bin `Abbas: `Ali bin Abu Talib came out of the house of Allah's Apostle during his fatal illness. The people asked, "O Abu Hasan (i.e. `Ali)! How is the health of Allah's Apostle this morning?" `Ali replied, "He has recovered with the Grace of Allah." `Abbas bin `Abdul Muttalib held him by the hand and said to him, "In three days you, by Allah, will be ruled (by somebody else ), And by Allah, I feel that Allah's Apostle will die from this ailment of his, for I know how the faces of the offspring of `Abdul Muttalib look at the time of their death. So let us go to Allah's Apostle and ask him who will take over the Caliphate. If it is given to us we will know as to it, and if it is given to somebody else, we will inform him so that he may tell the new ruler to take care of us." `Ali said, "By Allah, if we asked Allah's Apostle for it (i.e. the Caliphate) and he denied it us, the people will never give it to us after that. And by Allah, I will not ask Allah's Apostle for it."
USC-MSA web (English) Reference: Volume 5, Book 59, Number 728