Musnad Ahmad 25:297

Mu'adh ibn Mu'adh narrated to us, Hajib ibn 'Umar narrated to us, my paternal uncle al-Hakam ibn al-A'raj narrated to me, he said: I came to Ibn 'Abbas while he was reclining by Zamzam, so I sat with him, and he was an excellent companion. I said: "Inform me about the day of 'Ashura." He said: "About which of its affairs are you asking?" I said: "About fasting it; which day should I fast?" He said: "When you see the crescent of Muharram, then count. When you reach the morning of the ninth, then wake up fasting on it." I said: "Is that how Muhammad, may Allah bless him and grant him peace, used to fast it?" He said: "Yes."