Abu Sa'id al-Khudri (may Allah be pleased with him) narrates: I heard the Messenger of Allah (peace and blessings be upon him) say: "Whoever fasts Ramadan, knows its limits and bounds, and guards (it) as much as is within his power, his previous sins are wiped away."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8505