From Sahih Bukhari (Hadith 2741):
Narrated Al-Aswad:In the presence of `Aisha some people mentioned that the Prophet (ﷺ) had appointed `Ali by will as his successor. `Aisha said, "When did he appoint him by will? Verily when he died he was resting against my chest (or said: in my lap) and he asked for a wash-basin and then collapsed while in that state, and I could not even perceive that he had died, so when did he appoint him by will?
Reference: Bukhari 2741