Sayyiduna Abu Mas'ud (may Allah be pleased with him) says: We used to carry loads (for wages). Then one of us would give a large charity and it would be said of him that he did it to show off, and if someone gave half a sa' in charity it would be said to him that Allah Almighty is free of need of your charity. Then this blessed verse was revealed: "Those who criticise the believers who give charity voluntarily" up to "a painful punishment" [Surah al-Tawbah: 79].
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7748