It is narrated from Sayyiduna Qatadah bin al-Nu'man (may Allah be pleased with him) that he heard the Messenger of Allah (peace and blessings be upon him) say: "Whoever fasts the Day of Arafah, the sins of a year ahead of him and a year behind him are forgiven for him."
Hadith Referenceالمعجم الكبير للطبراني / باب القاف / 16056