The Levels of Fasting the Day of ‘Ashura’
The First Level: To fast the 9th , 10th and 11th. This is the highest rank. This is based on the Hadeeth reported by Ahmad “Fast a day before it and a day after it to oppose the Jews.” And because by fasting the three days, one will get the virtue of fasting three days of this blessed month.
The Second Level: Fasting the 9th and the 10th. This is based on the statement of the prophet (Salla Allahu ‘alayhi wa sallam) “If I remain alive till next year, I shall also observe fast on the 9th Muharram (meaning 9th and 10th) to be different form Jews.” He (Salla Allahu ‘alayhi wa sallam) loved opposing the Jews,
The Third level: To fast the 10th and the 11th.
The Fourth level: To fast the 10th alone. Some scholars said it is Mubaah(allowable) and some said it is Makrooh(disliked). Those who said it is allowable based it on the general statement of the Prophet (Salla Allahu ‘alayhi wa sallam): “It is expiation for the sins of the previous year.” Without the mention of the 9th. While those who said it is disliked, said so because the Prophet (Salla Allahu ‘alayhi wa sallam) said: “fast a day before it and a day after it.”
This obligates one to add a day to it for the sake of differentiation. Therefore opinion that states it is disliked is the strong opinion. Accordingly we see that one should fast the 9th or the 11th along with the tenth.
Source: Liqaa’ Al Baab Al-Maftooh (95) by Shaikh ibn Uthaimeen