Musnad Ahmad 25:376

'Ali ibn 'Asim narrated to us, Mu'awiyah ibn 'Amr ibn Ghallab informed us, from al-Hakam ibn 'Abd Allah ibn al-A'raj, who said: I was with Ibn 'Abbas in the house of the water-carrier (al-siqayah), and he was reclining on a cloak of his. He said: I said, "O Abu 'Abbas, inform me about 'Ashura'." He said, "About which of its affairs?" I said, "About fasting it." He said, "When you sight the new moon of al-Muharram, then count nine, and then wake up on the ninth day fasting." I said, "Is this how Muhammad, peace and blessings of Allah be upon him, used to fast it?" He said, "Yes."