Musannaf Ibn Abi Shayba 27:599

Waki’ narrated to us, from Sufyan, from Abu Ishaq, from Harithah ibn Mudharrib, who said: The letter of Umar was read to us: "To proceed: I have sent to you Ammar ibn Yasir as a commander, and Abdullah ibn Mas’ud as a teacher and a minister. They are among the noble ones of the companions of Muhammad, may Allah bless him and grant him peace. And I have preferred you over myself by [sending] Ibn Umm ‘Abd [Abdullah ibn Mas’ud]."