The reason why it is mustahabb to fast on Taasoo’aa’

Al-Nawawi (may Allaah have mercy on him) said: “The scholars – our companions and others – mentioned several reasons why it is mustahabb to fast on Taasoo’aa’:

1. the intention behind it is to be different from the Jews, who only venerate the tenth day. This opinion was reported from Ibn ‘Abbaas…
2. the intention is to add another day’s fast to ‘Aashooraa’. This is akin to the prohibition on fasting a Friday by itself, as was mentioned by al-Khattaabi and others.
3. To be on the safe side and make sure that one fasts on the tenth, in case there is some error in sighting the crescent moon at the beginning of Muharram and the ninth is in fact the tenth.”

The strongest of these reasons is being different from the People of the Book. Shaykh al-Islam Ibn Taymiyah (may Allaah have mercy on him) said: “The Prophet (peace and blessings of Allaah be upon him) forbade imitating the People of the Book in many ahaadeeth, for example, his words concerning ‘Aashooraa’: ‘If I live until the next year, I will certainly fast on the ninth day.’” (al-Fataawa al-Kubra, part 6, Sadd al-Dharaa’i’ al-Mufdiyah ila’l-Mahaarim )

Ibn Hajar (may Allaah be pleased with him) said in his commentary on the hadeeth “If I live until the next year, I will certainly fast on the ninth day”: “What he meant by fasting on the ninth day was probably not that he would limit himself to that day, but 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. This is also what we can understand from some of the reports narrated by Muslim.” (Fath, 4/245).