Musnad Ahmad 27:36

Isma'il narrated to us, Ayyub informed us, from Nafi', from Ibn 'Umar, that he said regarding Ashura: "The Messenger of Allah (peace and blessings of Allah be upon him) fasted it and commanded that it be fasted. But when Ramadan was made obligatory, it was abandoned." And 'Abdullah would not fast it, unless it coincided with his [customary] day of fasting.