Shaikh Muhammad Ibn Sālih Al-‛Uthaimeen (V) was asked: “Is it disliked to single out the 10th of Muharram* for fasting?” He replied: “Some scholars have said: It is disliked due to the saying of the Prophet (ﷺ): ❮Fast [along with it] a day before it or a day after it – differ from the Jews.❯ Other scholars have said: It is not disliked to fast just the 10th. However by doing so one loses out the reward that is attained with differing from the Jews – but what is correct is that it is not disliked to single out ‛Āshoorā’ with fasting.”
Sharh al-Mumti’ 6/468.
* i.e. The day of ‛Āshoorā’.