Musnad Ahmad 2207

It was narrated that al-Hakam bin 'Abdullah bin al-A'raj said: I was with Ibn 'Abbas (رضي الله عنه) in the house where they provide water, and he was using a cloak of his as a pillow. I said: O Ibn 'Abbas, tell me about 'Ashoora'. He said: What about it? I said: About fasting it. He said: When you see the new moon of Muharram, count nine (days), then fast on the ninth day. I said: Is that how Muhammad (ﷺ) fasted it? He said: Yes.