It is narrated from Sayyiduna Sahl bin Sa'd (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever fasts on the day of Arafah, the sins of two consecutive years will be forgiven for him."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5902