Sh. Muḥammad b. Ṣāliḥ al-ʿUthaymīn رَحِمَهُ اللهُ said:

“The scholars say, regarding the fasting of the Day of ʿĀshūrāʾ, that it is either [fasted] on its own, or he fasts with it the 9^th, or he fasts with it the 11^th. There is a fourth mode: which is to fast the 9^th, 10^th, and 11^th – so it is three days of the month.

It is better for the one who wants to fast two days, that he fasts the 9^th and the 10^th.”

See: Majmūʿ al-Fatāwá (20: 35, no. 395).