Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "The fasting of one Ramadan to the next Ramadan is an expiation for (the sins committed) between them."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5426