Tradition

The Messenger of Allah (s.a.w.a.) said, “There will be a dark conspiracy after me.
The only survivors will be ‘those who hold on to the firmest handle’ (Surah Baqarah (2):256).”

So the people asked the Prophet , “What is the firmest handle?”

The Prophet replied, “The Wilayat of the Master of the Successors.”

They asked the Prophet, “Who is the Master of the Successors?”

The Prophet replied, “The Commander of the Believers (Ameerul Momineen).”

They asked the Prophet , “Who is Ameerul Momineen?”

The Prophet replied, “The Master (Maula) of Muslims and their Imam after me.”

They asked the Prophet , “Who is the Master (Maula) of Muslims and their Imam after you?”

The Prophet replied, “My brother, Ali bin Abi Talib.”
Behaar al-Anwaar v 36 p 20 Ghaayah al-Maraam p 19