Musnad Ahmad 3188

Al-Hakam bin al-A'raj said: I came to Ibn 'Abbas (رضي الله عنه) when he was reclining on his cloak at Zamzam, and I said: Tell me about ‘Ashoora', what day should I fast it? He said: When you see the new moon of Muharram, count, and fast on the ninth day. I said: Is that how Muhammad (ﷺ) used to fast it? He said: Yes.