It was stated in the established hadith on the authority of the Messenger of God, may God bless him and grant him peace, regarding fasting on the day of Arafah. I hope that God will atone for the year before it and the year after it. Muslim included it from the hadith of Abu Qatada

Whoever fasts this day will receive a great share. From what God gave to His Prophet, may God bless him and grant him peace, in his saying, “May God forgive you for your previous sins and those that will follow.” The Messenger of God, may God bless him and grant him peace, continued his entire life in ruling like a fasting person on the Day of Arafah.