Fasting on the 9th and 10th of Muharram.

الجواب حامدا و مصلیا

According to the below mentioned narration, the fast of the 10th of Muharram was made compulsory before the fast of the holy month of Ramazan. Afterwards, when the fast of Ramazan was made compulsory, it was not compulsory anymore to fast on the 10th of Muharram. Now it is Sunnat and recommended to fast on this day, not a compulsion. Whereas, the command was given to fast on the 9th or 11th of Muharram along with it (i.e. the 10th of Muharram) to not resemble the ways of the Jews (who fasted only on the 10th of Muharram).
With regards to the virtue of fasting on the 10th of Muharram, the noble Prophetﷺ said, “I am hopeful from my Lord that the fast of Ashura becomes a means of atonement for the sins committed the year before it as well as the year after it.”
And Allah knows best.