Abu Dawud Book 25, Hadith Number 3987.

Narated By Abu Sa’id al-Khudri : The Apostle of Allah (PBUH) related a tradition in which he mentioned the words “Jibril and Mikal” and he pronounced them “Jibra’ila wa Mika’ila.”

Abu Dawud said: Khalaf said: I did not put the pen aside from writing letters (huruf) for forty years; nothing tired me (or made me incapable of writing), even Jibril and Mika’il did not tire me.