Musnad Abu Ya'la al-Mawsili 42:159

'Abd al-A'la ibn Hammad narrated to us, Hammad narrated to us, from Abu Harun al-'Abdi, from Abu Sa'id al-Khudri, that the Messenger of Allah (may Allah bless him and grant him peace) commanded the fasting of 'Ashura, and he used to not fast it.