When (fasting) in Ramadan was (made) obligatory, he abandoned it (the practice of observing fast on Ashura).
Sahih Muslim Book 13, Hadith 155
← Previous Hadith When al-Ash'ath b. Qais entered the house of 'Abdullah he was having his breakfast. He ('Abdullah b. Umar) said: Abd Muhammad (al-Asha'th), come near to the breakfast. Thereupon he said: Is not today the day of 'Ashura? He ('Abd al-Rahman) said: Do you know what the day of 'Ashura is? He said: What is it? He said: It is a day on which the Messenger of Allah (ﷺ) used to observe fast before the (fasting) in the month of Ramadan (became) obligatory. But when it became obligatory the (fasting of 'Ashura) was abandoned (as compulsory). Abu Kuraib said: He (the Holy Prophet) abandoned it.

Sahih Muslim Book 13, Hadith 154 Next Hadith → Abu Muhammad, come near and dine. Upon this he said: I am fasting. Thereupon he said: We used to observe fast and then (this practice) was abandoned.

Sahih Muslim Book 13, Hadith 156