Musnad Ahmad 1153:2088

Uthman ibn Umar narrated to us, he said: Yunus informed us, from al-Zuhri, from Urwah, from Aisha: "The Prophet, peace and blessings of Allah be upon him, used to command the fasting of Ashura before Ramadan was revealed. Then, when Ramadan was made obligatory, whoever wished fasted, and whoever wished did not fast."