Tradition

The Prophet (s.a.w.a.) was in Umme-a-Salma’s house when an angel, who had twenty heads, descended upon him. Each one of the angel’s heads had one thousand tongues, and he was praising Allah with each tongue in a different language. His wings were larger than all of the seven heavens and the
seven earths.

The Prophet (s.a.w.a.) thought that he was Jibraeel so the Prophet (s.a.w.a.).said to him – I have never seen you in this form.

The angel said – I am not Jibraeel. I am Sarsaeel. Allah sent me to you to marry one light to another.

Then the Prophet (s.a.w.a.) asked him – Who do you mean?

The angel said – Your daughter, Fatima, to Ali b. Abi Talib.

So the Prophet (s.a.w.a.) married Fatima to Ali with Jibraeel, Mikaeel, Israfeel, and Sarsaeel as witnesses.

Then the Prophet (s.a.w.a.) looked and saw the following written between the shoulders of Sarsaeel:

LA ILAHA ILLALLAH. MOHAMMAD IS THE MESSENGER OF ALLAH AND IS THE PROPHET WHO BRINGS MERCY ON PEOPLE. ALI IS THE DECISIVE PROOF THAT ALLAH USES ON HIS CREATURES.

Then the Prophet (s.a.w.a.) asked Sarsaeel – How long have you had this written between your shoulders?

Sarsaeel replied – Twelve thousand years before Allah created Adam.
Al-Manaaqib p 245 Behaar al-Anwaar v 43 p 123 Al-Muhtazar p 133