Ali called upon the people to testify, so five or six of the companions of the Prophet ﷺ stood up and bore witness that the Messenger of Allah ﷺ said, "Whoever I am his master, Ali is his master."
نشد علي الناس، فقام خمسة أو ستة من أصحاب النبي ﷺ فشهدوا أن رسول الله ﷺ قال: من كنت مولاه فعلي مولاه.
Reported by Ahmad (23107) and Nasai in Khasais Ali (86), both by way of Shubah, from Abu Ishaq, who said, I heard Saeed bin Wahb say, then he related it. Its chain is sahih. His statement, "Ali appealed to the people," means whoever had heard the Messenger of Allah ﷺ say, "Whoever I am his master, Ali is his master." This hadith has been reported through other chains from Ali, and this one is the most sahih.