It is narrated from Sayyiduna Zayd bin Arqam (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) was asked about fasting on the Day of Arafah, and he (peace and blessings be upon him) said: "It becomes an expiation for the sins of this year and of the year that comes after it."
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 5070