Musnad Ahmad 788:185

Abu Usamah narrated to us, Abu al-Umays narrated to me, from Qays ibn Muslim, from Tariq ibn Shihab, from Abu Musa, who said: The day of Ashura was a day that the Jews would fast and take as a festival. The Messenger of Allah (peace and blessings of Allah be upon him) said: "You [also] fast it."