Musannaf Ibn Abi Shayba 7:60

Ibn Idris narrated to us, from Layth, from Talha, from Yasar ibn Numayr, who said: Umar said to me, "I am one of the commanders of the Muslims. So if you see that I have sworn an oath and I do not fulfill it, then feed ten poor people on my behalf; for every poor person, half a sa' of wheat, or a sa' of barley, or a sa' of dates."