(50) باب مَا جَاءَ عَاشُورَاءُ أَىُّ يَوْمٍ هُوَ
Al-Hakam bin Al-Arfaj said:
"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.'"
Reference: Jami` at-Tirmidhi 754
In-book reference: Book 8, Hadith 73
English translation: Vol. 2, Book 3, Hadith 754