"The Messenger of Allah ordered fasting the tenth day for the Day of Ashura."
Jami` at-Tirmidhi Book 8, Hadith 74
← Previous Hadith "I met up with Ibn Abbas while he was reclining on his Rida at the Zamzam (well). So I said: 'Inform me about the Day of Ashura. On which day is it fasted?' He said: 'When you see the crescent of Al-Muharram, then count, then fast on the morning of the ninth day.'" He said: 'I said: Is this how Muhammad fasted it?' He said: 'Yes.'"

Jami` at-Tirmidhi Book 8, Hadith 73 Next Hadith → "I did not see the Prophet fasting at all during the ten."

Jami` at-Tirmidhi Book 8, Hadith 75