Musnad Ahmad 25:681

'Affan narrated to us, Hajib bin 'Umar Abu Khushaynah, the brother of 'Isa al-Nahwi, narrated to us, he said: Al-Hakam bin al-A'raj narrated to us, he said: I sat with Ibn 'Abbas, and he was reclining on his garment by the well of Zamzam, so I sat with him, and he was an excellent companion. I asked him about 'Ashura? He said: "About which of its aspects are you asking?" I said: About fasting it. He said: "When you see the crescent of Muharram, then count, and when you reach the morning of its ninth, then fast that day." I said: Is this how Muhammad, may Allah bless him and grant him peace, used to fast it? He said: "Yes."