Imam Muhammad Baqir (a.s.) said – On the Night of Ascent (Me’raj), Allah informed the Prophet (s.a.w.a.) – O Muhammad! Your prophethood is coming to an end and your death is approaching. So who will lead your nation after you?
The Prophet (s.a.w.a.) replied – O My Lord! I have tested all of Your creation and I have not found anyone who loves me more intensely than Ali b. Abi Talib (a.s.).
Allah replied – O Muhammad! Then inform Ali (a.s.) that he is the flag of guidance and the Imam of My Friends. Ali (a.s.) is the light of those who obey Me. Bashaarah al-Mustafa Le Shiah al-Murtaza v 2 p 151