Musannaf Ibn Abi Shaybah 9631 — The day of Ashura: which day it is

Al-Hakam bin al-A'raj said: I came to Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) while he was lying with his cloak as a pillow at Zamzam. I said: "Tell me about the fast of Ashura." He said: "When you see the new moon of Muharram, count the days, then fast on the ninth." I said: "Is this how the Prophet (peace and blessings be upon him) used to fast it?" He said: "Yes."