Ali ibn Abi Talib is the Imam after our Prophet (May Allah's blessing and peace be on him and his descendants). The Holy Prophet addressing Imam Ali said: "Ali, you are my brother, the heir of my knowledge, my successor after me and payer of my debt. You are to me as Aaron was to Moses, but there shall be no prophet after me." The Holy Prophet also said: "Greet Ali as the Commander of the Faithful, listen to him and obey him. Get knowledge from him but do not teach him." On another occasion the Holy Prophet said: "This Ali is the master of him whose master I am. May Allah be friendly to him who is friendly to Ali and be hostile to him who is hostile to Ali."
source : http://www.islamicecenter.com/e-library/logic_islamic_rules/logic_islamic_rules_makarem_subhani_03.html
