Musnad Ahmad 25:1353

Waki’ narrated to us, Hajib ibn ‘Umar narrated to us, he heard it from al-Hakam ibn al-A’raj, who said: I arrived at Ibn ‘Abbas while he was using his garment as a pillow at Zamzam. I said: Inform me about ‘Ashura, which day should I fast? He said: "When you see the crescent of Muharram, then count, and wake up on the ninth as one who is fasting." I said: "Is that how Muhammad, peace and blessings be upon him, used to fast it?" He said: "Yes."