Al-Shaafa’i and his companions, Ahmad, Ishaaq and others said: It is mustahabb [recommended] to fast both the ninth and the tenth, because the Prophet (peace and blessings of Allaah be upon him) fasted the tenth and intended to fast the ninth.
Based on this, there are different ways of fasting ‘Ashoora’, the least of which is to fast the tenth only, but it is better to fast the ninth as well. The more one fasts in Muharram, the better.
Ibn Hajar (may Allaah have mercy on him) said, commenting on the hadeeth “If I live until next year I will certainly fast the ninth”: His concern to fast the ninth may be understood as meaning that he would not limit himself to that, rather that he would add it to the tenth, either to be on the safe side, or to be different from the Jews and Christians, which is more likely to be correct; this is what may be understood from some of the reports narrated by Muslim.”