Musnad Ahmad 25:101

Sufyan narrated to us, he said: Ubaydullah ibn Abi Yazid informed me - seventy years ago - he said: I heard Ibn Abbas saying: "I have not known the Messenger of Allah, may Allah bless him and grant him peace, to have fasted a day seeking its merit over other days, except for the day of Ashura." And Sufyan said another time: "Except for this day, meaning Ashura, and this month, the month of Ramadan."