Musannaf Ibn Abi Shaybah

Hazrat Abu Ishaq (may Allah have mercy on him) narrates that Hazrat Haritha bin Mudarrib (may Allah have mercy on him) said: The letter of Hazrat Umar (may Allah be pleased with him) was read out to us. In it was written: After praise and salutations, indeed I have sent Hazrat Ammar bin Yasir (may Allah be pleased with him) to you as your leader, and Hazrat Abdullah bin Mas’ud (may Allah be pleased with him) as a teacher and minister. Both of them are among the noble companions of the Noble Prophet (peace and blessings of Allah be upon him). And in the matter of Ibn Umm Abd (may Allah be pleased with him), I have given preference to others over myself.