Sayyiduna Abu Mas'ud (may Allah be pleased with him) said: We used to work as labourers, and one of us would bring a little charity and another would bring half a sa', and some of the hypocrites would taunt us. Then this verse was revealed: "Those who taunt the believers who give charity willingly concerning (their) charities" [Surah al-Tawbah: 79] (to the end of the verse).
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14607