Musnad Ahmad 1937

Sufyan said: "Ubaidullah bin Abu Yazeed told me seventy years ago. I heard Ibn "Abbas say: I did not see the Messenger of Allah (ﷺ) fast any day, seeking its virtue and superiority over other days, apart from the day of "Ashoora" - and on one occasion Sufyan said. Except this day, meaning "Ashoora" - and this month, the month of Ramadan.