Muslim

Hadith Commentary:
This hadith mentions the arrival of the Prophet (sallallahu alayhi wa sallam) in Madinah.
The meaning of the chapter is derived from this.
Later, the Messenger of Allah (sallallahu alayhi wa sallam) said that any Muslim who fasts on ‘Ashura should also fast on either the ninth or the eleventh day, that is, he should fast one additional day, in order to oppose the Jews.
Now, observing this fast is Sunnah.

Hadith Commentary:

In Musnad Ahmad, it is narrated from Ibn Abbas (radi Allahu anhu) in a marfu‘ (attributed to the Prophet) narration that: "Fast on the day of ‘Ashura, and oppose the Jews in this; fast a day before it or a day after it." That is, the Messenger of Allah (sallallahu alayhi wa sallam) said: Fast on the day of ‘Ashura, and to differ from the Jews, combine with it a fast on the day before or after.

Al-Qurtubi said: ‘Ashura is a term derived from ‘Ashirah for the purpose of emphasis and magnification. Originally, it is an adjective for the tenth night, because it is taken from ‘ashr, which is the name of the decade, and the day is annexed to it. So when it is said "the day of ‘Ashura," it is as if it is said "the day of the tenth night." However, when they deviated from using it as an adjective, the nominal usage became predominant, so they dispensed with the described noun and omitted "night," and thus this word became a proper noun for the tenth day. (Fath)

That is, Qurtubi said that the word ‘Ashura is for emphasis and magnification, and is derived from ‘Ashirah. Whenever the word ‘Ashura is used, it refers to the night of the tenth of Muharram.