"I heard Mu'awiyah say on the day of 'Ashura when he was on the Mindar: O people of Al-Madinah, where are your scholars? I heard the Messenger of Allah say on this day: "I am fasting, so whoever wants to fast let him do so."
Sunan an-Nasa'i Book 22, Hadith 282
← Previous Hadith he heard Ibn 'Abbas, when he was asked about the fast of 'Ashura' say: "I do not know that the Prophet fasted any day because of its virtue, except this day" meaning the month of Ramadan and the day of Ashura.'

Sunan an-Nasa'i Book 22, Hadith 281 Next Hadith → "One of the wives of the Prophet said: "The Prophet used to fast on the day of 'Ashura, nine days of Dhul-Hijjah and three days each month: The first Monday of the month, and two Thursday."

Sunan an-Nasa'i Book 22, Hadith 283