Harithah bin Mudarrib narrates that Sayyiduna Umar (may Allah be pleased with him) wrote to the people of Kufah: "I have sent Ammar (may Allah be pleased with him) as governor and Abdullah bin Mas'ud (may Allah be pleased with him) as teacher and minister. These two are among the noblest of the Companions of Muhammad (peace and blessings be upon him), from the people of Badr and the people of Uhud. So follow them both and listen to what they say. I have preferred you over myself in the matter of Abdullah."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 8446