Musannaf Ibn Abi Shayba 7:50

Abu Khalid al-Ahmar narrated to us, from al-A'mash, from Shaqiq, from Yasar ibn Numayr, who said: Umar said to me: "I swear that I will not give certain people anything, then it appears to me [that I should] and I give them. So if I do that, then feed ten poor people on my behalf, [giving] between every two poor people a sa' of wheat or a sa' of dates for each poor person."