ʿAlī ibn ʿAbdullāh narrated to us; Sufyān narrated to us; Ayyūb al-Sakhtiyānī narrated to us; from the son of Saʿīd ibn Jubayr; from his father;

from Ibn ʿAbbās (may Allāh be content with them), that: When the Prophet ﷺ came to Madinah, he found the people fasting on a day — meaning ʿĀshūrā'. They said: “This is a great day. It is the day on which Allāh saved Mūsā (Moses) and drowned the family of Pharaoh. So, Mūsā fasted it in gratitude to Allāh.” The Prophet ﷺ said: “I have more right to Mūsā than they do.”

So, he fasted it and commanded that it be fasted.