It is narrated from the Prophet of Islam (PBUH) who said:

O’ People, I will go out of this world before you and you will join me beside the Pond of Kawthar. Then I will ask you about Thaqalayn (the holy Quran and my Ahlul Bayt (AS)). I left two things with you in trust; the Book of God and my Ahlul Bayt (AS). Do not get in front of them nor fall behind. O’ people, do not become disbelievers (Kafir) after me, and do not kill each other. Beware that Ali Ibn Abitaleb (AS) is my brother and my successor. And as I battled in the way of the Quran’s descent (Nuzul), Ali (AS) will battle in the way of commentary (Tafseer) of the Quran [1].

[1] A’ayanu Shia, Vol. 2, Page 226.

Source: en.shafaqna.com