Sahih Muslim · Hadith 4318

وحدثنا يحيى بن يحيى، وأبو بكر بن أبي شيبة - واللفظ ليحيى - قال أخبرنا إسماعيل ابن علية، عن ابن عون، عن إبراهيم، عن الأسود بن يزيد، قال ذكروا عند عائشة أن عليا كان وصيا فقالت متى أوصى إليه فقد كنت مسندته إلى صدري - أو قالت حجري - فدعا بالطست فلقد انخنث في حجري وما شعرت أنه مات فمتى أوصى إليه.

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?

Gradings are classical jarḥ-wa-taʿdīl, name-matched to a rijāl index (not individually re-verified).