Sayyiduna Abu Qatadah al-Ansari (may Allah be pleased with him) narrates 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 the past year and the coming year."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8380