Musnad Ahmad 2528

Al-Hakam bin al-A'raj said: I sat with Ibn 'Abbas (رضي الله عنه) when he was using his cloak as a pillow at the well of Zamzam. I sat with him, and what good company he was. I asked him about ‘Ashoora’ and he said: What about it? I said: Fasting it. He said: When you see the new moon of Muharram, count, and when it is the ninth day, fast that day. I said: Is this how Muhammad (ﷺ) used to fast it? He said: Yes.