Musannaf Ibn Abi Shayba 28:392

Abu Usama narrated to us, from Abdullah ibn al-Walid, from Asim ibn Abi al-Najud, from the son of Khuzayma ibn Thabit, who said: When Umar would appoint a man, he would have a group of the Ansar and others bear witness against him. He would say: "I have not appointed you over the blood of the Muslims, nor over their honor, but I have appointed you over them to distribute among them with justice and to establish the prayer among them." And he would stipulate upon him that he should not eat refined flour, nor wear thin clothing, nor ride a pack horse, nor close his door against the needs of the people.