Musnad Abu Ya'la al-Mawsili 252:39

Abu Bakr ibn Abi Shaybah narrated to us, Mu'awiyah ibn Hisham narrated to us, from Abu Hafs al-Ta'ifi, from Abu Hazim, from Sahl ibn Sa'd, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever fasts the day of Arafah, two consecutive years will be forgiven for him."