Musannaf Ibn Abi Shaybah

Hazrat Hakam bin A'raj says that I went to the service of Hazrat Abdullah bin Abbas (may Allah be pleased with them both); he was sitting leaning against the well of Zamzam. I said, "Tell me about the fast of Ashura." He replied, "When you see the crescent of Muharram, start preparing for that day. Then fast on the ninth day." I asked, "Did the Messenger of Allah (peace and blessings be upon him) use to fast on that day?" He replied, "Yes."