It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that he (peace and blessings be upon him) said: "And a man who gives charity secretly, so much so that even his left hand does not know what his right hand has spent."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7837