And feed with food the needy wretch, the orphan and the prisoner, for love of Him) [76:8]. ‘Ata’ reported that Ibn ‘Abbas said: “It happened that ‘Ali ibn Abi Talib hired himself one night to water some palm-trees in exchange for some barley. The following morning, he collected his barley and grinded a third of it out of which they made something to eat, called al-Khazirah. When it was cooked, a poor man came begging from them, and so they gave him the food they had prepared. They then prepared a third of the remaining barley and when it was cooked, an orphan came begging from them, and they gave him the food. They then went and prepared what was left of that barley, but when the food was cooked, a prisoner from among the idolaters came to them and they fed him that food and spent the rest of the day without eating anything. This verse was revealed about this incident”.
Asbab Al-Nuzul by Al-Wahidi , trans. Mokrane Guezzou