Ibn ʿAbbās [رضي الله عنه] said:

❝The Prophet [ﷺ] came to al-Madīnah and saw the Jews fasting on the day of ʿĀshūrā. He asked, ‘What is this?’ They responded, ‘This is a virtuous day, as this is the day when Allāh saved the Children of Isrā’īl from their enemy. Thus, Mūsá fasted on this day.’ He said, ‘We have more right to Mūsá than you.’ So he fasted on this day and ordered the [people] to fast.❞

● [Sahīh Bukharī, (No. 2004)]