The Messenger of Allah ﷺ commanded us to fast on the day of Ashura and said, "It is a day on which the Jews used to fast."
حدثنا حسن حدثنا ابن لهيعة أخبرنا أبو الزبير
أمرنا رسول الله ﷺ بيوم عاشوراء أن نصومه وقال هو يوم كانت اليهود تصومه
No references are available.
Its isnad is hasan, and it is found in Bukhari 3/31, in the Book of Fasting, chapter on the obligation of fasting Ramadan, and in Abu Dawud 1/569, in the Book of Fasting, chapter on fasting the day of Ashura.