From Sahih Muslim (Hadith 4231):
Aswad b. Yazid reported:It was mentioned before A'isha that will had been made (by the Holy Prophet) in favour of 'Ali (as the Prophet's first caliph), whereupon she said: When did he make will in his favour? I had been providing support to him (to the Holy Prophet) with my chest (or with my lap). He asked for a tray, when he fell in my lap (relaxing his body), and I did not realise that he had breathed his last. When did he make any will in his ('Ali's) favour?
Reference: Muslim 4231