Sulaim Ibn Qais said regarding Surah Nisa (4): 59: Obey Allah and obey the Apostle and those in authority from among you. O Amirul Mumineen, please give his name.
Ali (a.s.) informed: He is the same person whom the Holy Prophet (s.a.w.a.) appointed in Ghadheer Khum and showed him to everyone and told them that he has authority over every soul, then commanded that those present to convey the message to the absent ones.
Sulaim said: Is that you, O Ameerul Mumineen?
He (a.s.) said: I am the first of them and the best of them. Then my son Hasan is after me, who has authority over believers, then my son Husain, who is after him, who has authority over believers. Then there are all successors of the Holy Prophet (s.a.w.a.) at the Fountain, one by one.
At that time that person stood next to Ali (a.s.) and kissed his forehead and then said: You have made it clear and solved my problem and whatever I had in my heart you have cleared it. Kitaab Sulaym Ibn Qais v 2 p 616-617