Musannaf Ibn Abi Shayba 4:514

Waki’ ibn al-Jarrah narrated to us, from Hajib ibn ‘Umar, from al-Hakam ibn al-A’raj, who said: I reached Ibn ‘Abbas while he was using his cloak as a pillow at Zamzam. I said: "Inform me about fasting the day of ‘Ashura." He said: "When you see the crescent of Muharram, then count, and wake up fasting on the ninth." I said: "Is this how Muhammad, the Messenger of Allah (may Allah bless him and grant him peace), used to fast it?" He said: "Yes."