'Ashurah was a day on which we used to fast in pre-Islamic days. When (fasting of) Ramadan was prescribed, the Messenger of Allah (ﷺ) said: This is one of the days of Allah ; he who wishes may fast on it.
Sunan Abi Dawud Book 14, Hadith 131
← Previous Hadith The Quraish used to fast on the day of 'Ashurah in pre Islamic days. The Messenger of Allah (ﷺ) would fast on it in pre-Islamic period. When the Messenger of Allah (ﷺ) came to Medina, he fasted on it and commanded to fast on it. When the fast of Ramadan was prescribed, that became obligatory, and (fasting on) 'Ashurah was abandoned. He who wishes may fast on it and he who wishes may leave it.

Sunan Abi Dawud Book 14, Hadith 130 Next Hadith → When the Prophet (ﷺ) came to Medina, he found the Jews observing fast on the day of 'Ashurah; so they were asked about it (by the Prophet). They said: This is a day on which Allah gave Moses domination over Pharaoh. We fast on it out of reverence to him. The Messenger of Allah (ﷺ) said: We have a closer connection with Moses than you have. He then gave orders that it should be observed.

Sunan Abi Dawud Book 14, Hadith 132