Sahih Bukhari
Military Expeditions led by the Prophet (pbuh) (Al-Maghaazi)
Volume: 005 / Book Number: 059 / Hadith Number: 736
Narrated Al-Aswad:
It was mentioned in the presence of 'Aisha that the Prophet had appointed 'Ali as successor by will. Thereupon she said, "Who said so? I saw the Prophet, while I was supporting him against my chest. He asked for a tray, and then fell on one side and expired, and I did not feel it. So how (do the people say) he appointed 'Ali as his successor?"