the Prophet said: "Fast the Day of Ashura, for indeed I anticipate that Allah will forgive (the sins of) the year before it."
Jami` at-Tirmidhi Book 8, Hadith 71
← Previous Hadith "Ibn Umar was asked about fasting (the Day of) Arafah (at Arafat). He said: 'I performed Hajj with the Prophet, and he did not fast it, and with Abu Bakr, and he did not fast it, and with Umar, and he did not fast it, and with Uthman, and he did not fast it. I do no fast it, nor order it nor forbid it."

Jami` at-Tirmidhi Book 8, Hadith 70 Next Hadith → "Ashura was a day that the Quraish used to fast during Jahiliyyah, and the Messenger of Allah used to fast it. But when (the fast of) Ramadan became obligatory, the Ramadan was the required and Ashura was left. So whoever wanted to, he fasted it, and whoever wanted to, he left it."

Jami` at-Tirmidhi Book 8, Hadith 72