Musnad Ahmad 25:1534

Isma'il narrated to us, Yunus informed us, from al-Hakam ibn al-A'raj, who said: I asked Ibn Abbas about the day of Ashura. He said: "When you see the crescent of Muharram, then count. When you reach the morning of the ninth, then wake up fasting." Yunus said: It was conveyed to me from al-Hakam that he said: "Did Muhammad, may Allah bless him and grant him peace, fast like that?" He said: "Yes."