It is narrated from Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Fasting on the Day of Arafah is an expiation for the past year and the coming year."
Hadith Referenceالمعجم الأوسط / باب الألف / 2065
Hadith Takhrij«أخرجه عبد بن حميد فى «مسنده» برقم: 967، وابن حجر فى «المطالب العالية» برقم: 1091»