Mukhtasar Muslim

It is narrated from Hakam bin A'raj, he said: I came to Sayyiduna Ibn Abbas (may Allah be pleased with them), and he was sitting by the side of Zamzam, reclining on his cloak. So I said to him: Tell me about the fasting of 'Ashura. He said: When you see the crescent of Muharram, keep counting the days. Then when it is the ninth (9th) day, fast on that day. I said: Did the Messenger of Allah (peace and blessings be upon him) do the same? He said: Yes.