Sayyiduna Abu Qatadah (may Allah be pleased with him) narrates: A man asked the Prophet (peace and blessings be upon him). He said: O Messenger of Allah (peace and blessings be upon him)! What is your opinion about a person who fasts on the day of Ashura? The Prophet (peace and blessings be upon him) said: It is equal to fasting for a whole year. He asked: What is your opinion about a person who fasts on the day of Arafah? The Prophet (peace and blessings be upon him) said: It wipes out the sins of the previous year and the coming year.
Hadith Referenceصحیح ابن حبان / كتاب الصوم / 3631
Hadith Gradingفضيلة الشيخ الإمام محمد ناصر الدين الألبانيصحيح - «صحيح أبي داود» (2096): م.فضيلة الشيخ العلّامة شُعيب الأرناؤوطإسناده صحيح على شرط مسلم