Sayyiduna Abu Mas'ud (may Allah be pleased with him) said: We used to work as labourers for charity, and one of us would bring a sa', and some of the hypocrites would taunt them. Then this verse was revealed: "Those who taunt the believers who give charity willingly concerning (their) charities" [Surah al-Tawbah: 79].
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14608