I heard the Messenger of Allah ﷺ say, "Whoever fasts on the day of Arafah will have his sins forgiven for the year before it and the year after it."
حدثنا هشام بن عمار حدثنا يحيى بن حمزة عن إسحق بن عبد الله عن عياض بن عبد الله عن أبي سعيد الخدري
سمعت رسول الله ﷺ يقول من صام يوم عرفة غفر له سنة أمامه وسنة بعده
No references are available.
[Its chain is daif] Tabarani narrated it (al-Kabir: 5/19), from the hadith of Hisham with this wording. See no. 345 for its defect. The preceding hadith serves in its place. It has also been said that Zayd bin Aslam narrated it from Iyad with this wording. And Allah knows best.