The Memoirs of Amirul Mu’minin

When the noble verse: “And admonish your nearest kinsmen.” [Al-Shu’arā - 214] was sent down to the Messenger of Allah, he called me: “O Ali!” he said. “I have been told to invite [and urge] my next of kin to worship the Unique God, and warn them of the Divine Torment.

On the other hand, I know well enough, that if I were to let them know of this Mission, I shall receive a negative response from them… That is why I remained silent so far, and told nothing about it to anyone… In fact, I was waiting for a propitious moment, until the Archangel Gabriel came down [from Heaven] and announced to me.
“O Muhammad! If you do not execute your Mission, you shall suffer the Divine Torment…!”