Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said that once Sayyiduna Ali (may Allah be pleased with him) came out from the Prophet (peace and blessings be upon him) while he (peace and blessings be upon him) was in his final illness. The people asked: O Abu al-Hasan! How is the Prophet (peace and blessings be upon him)? He said: 'Alhamdu lillah' (praise be to Allah), "he is better." Sayyiduna Abbas bin Abd al-Muttalib (may Allah be pleased with him) took Sayyiduna Ali (may Allah be pleased with him) by the hand and said: By Allah, after three (days) you will be the one carrying the stick (i.e. under someone else's authority). I think that the Prophet (peace and blessings be upon him) will pass away in this illness, for I know the faces of Banu Abd al-Muttalib at the time of death. So let us go to the Prophet (peace and blessings be upon him) and ask him about the leadership: if it is to be among us, we will know it, and if it is to be among others, we will speak to him so that he gives instructions concerning us. Sayyiduna Ali (may Allah be pleased with him) said: "I will never ask him (peace and blessings be upon him), because if he (peace and blessings be upon him) refuses it, the people will never give it to us after that."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16574