Ibn 'Abbas, may Allah be pleased with them both, said:
"I never saw the Messenger of Allah, upon whom be peace, so keen to fast any day and give it priority over any other than this day, the day of 'Ashura, and this month, meaning Ramadhan." [Reported by al-Bukhari, 1867]
The meaning of his being keen was that he intended to fast on that day in the hope of earning the reward for doing so.
The Prophet, peace be upon him, said:
"For fasting the day of 'Ashura, I hope that Allah will accept it as expiation for the year that went before." [Reported by Muslim, 1976]
This is from the bounty of Allah towards us: for fasting one day He gives us expiation for the sins of a whole year. Indeed Allah is the Owner of Great Bounty.
An-Nawawi, may Allah have mercy on him, said:
"'Ashura and Tasu'a are two elongated names [the vowels are elongated] as is stated in books on the Arabic language. Our companions said: 'Ashura is the tenth day of Muharram and Tasu'a is the ninth day. This is our opinion, and that of the majority of scholars. This is the apparent meaning of the ahadith and is what we understand from the general wording. It is also what is usually understood by scholars of the language." [Al-Majmu']
'Ashura is an Islamic name that was not known at the time of Jahiliyyah. [Kashshaf al-Qina', Part 2: Sawm Muharram]
Ibn Qudamah, may Allah have mercy on him, said:
"'Ashura is the tenth day of Muharram. This is the opinion of Sa'id ibn al-Musayyib and al-Hasan. It was what was reported by Ibn 'Abbas, who said: 'The Messenger of Allah, peace be upon him, commanded us to fast 'Ashura, the tenth day of Muharram.' [Reported by al-Tirmidhi, who graded it a sahih hasan hadith] It was reported that Ibn 'Abbas said: 'The ninth,' and reported that the Prophet, peace be upon him, used to fast the ninth. [Reported by Muslim] 'Ata reported that he said, 'Fast the ninth and the tenth, and do not be like the Jews.' If this is understood, we can say on this basis that it is mustahabb (encouraged) to fast on the ninth and the tenth, for that reason. This is what Ahmad said, and it is the opinion of Ishaq."
'Abdullah ibn 'Abbas, may Allah be pleased with them both, said:
"When the Messenger of Allah, peace be upon him, fasted on 'Ashura and commanded the Muslims to fast as well, they said, 'O Messenger of Allah, it is a day that is venerated by the Jews and Christians.' The Messenger of Allah, peace be upon him, said:
'If I live to see the next year, insha'Allah, we will fast on the ninth day too.'
But it so happened that the Messenger of Allah, peace be upon him, passed away before the next year came." [Reported by Muslim, 1916]
Ash-Shafi'i and his companions, Ahmad, Ishaq and others said:
"It is mustahabb to fast on both the ninth and tenth days, because the Prophet, peace be upon him, fasted on the tenth, and intended to fast on the ninth."
On this basis it may be said that there are varying degrees of fasting 'Ashura, the least of which is to fast only on the tenth and the best of which is to fast the ninth as well. The more one fasts in Muharram, the better it is.
An-Nawawi, may Allah have mercy on him, said:
"The scholars – our companions and others – mentioned several reasons why it is mustahabb to fast on Tasu'a:
– the intention behind it is to be different from the Jews, who only venerate the tenth day. This opinion was reported from Ibn 'Abbas ...
– the intention is to add another day's fast to 'Ashura. This is akin to the prohibition on fasting a Friday by itself, as was mentioned by al-Khattabi and others.
– 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 ul-Islam Ibn Taymiyyah, may Allah have mercy on him, said:
"The Prophet, peace be upon him, forbade imitating the People of the Book in many ahadith, for example, his words concerning 'Ashura:
'If I live until the next year, I will certainly fast on the ninth day.' " [Al-Fatawa al-Kubra]
Ibn Hajar, may Allah be pleased with him, said in his commentary on this hadith:
"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]
Shaykh ul-Islam said:
"Fasting on the day of 'Ashura is an expiation for a year, and it is not makruh to fast only that day ... " [Al-Fatawa al-Kubra].
In Tuhfat al-Muhtaj by Ibn Hajar al-Haytami, it says:
"There is nothing wrong with fasting only on 'Ashura."
At-Tahawi, may Allah have mercy on him, said:
"The Messenger of Allah, peace be upon him, allowed us to fast on 'Ashura and urged us to do so. He did not say that if it falls on a Saturday we should not fast. This is evidence that all days of the week are included in this. In our view – and Allah knows best – it could be the case that even if this is true (that it is not allowed to fast on Saturdays), it is so that we do not venerate this day and refrain from food, drink and intercourse, as the Jews do. As for the one who fasts on a Saturday without intending to venerate it, and does not do so because the Jews regard it as blessed, then this is not makruh ... " [Mushkil al-Athar]
The author of Al-Minhaj said:
"'It is disliked (makruh) to fast on a Friday alone ... ' But it is no longer makruh if you add another day to it, as mentioned in the sahih report to that effect. A person may fast on a Friday if it coincides with his habitual fast, or he is fasting in fulfilment of a vow, or he is making up an obligatory fast that he has missed, as was stated in a sahih report."
Al-Sharih said in Tuhfat al-Muhtaj:
"'If it coincides with his habitual fast' – i.e., such as if he fasts alternate days, and a day that he fasts happens to be a Friday. 'If he is fasting in fulfilment of a vow, etc.' – this also applies to fasting on days prescribed in Shari'ah, such as 'Ashura or 'Arafah."
Al-Bahuti, may Allah have mercy on him, said:
"It is makruh to deliberately single out a Saturday for fasting, because of the hadith of 'Abdullah ibn Bishr, who reported from his sister: 'Do not fast on Saturdays except in the case of obligatory fasts' [Reported by Ahmad with a jayyid isnad and by al-Hakim, who said it was according to the conditions of al-Bukhari], and because it is a day that is venerated by the Jews, so singling it out for fasting means being like them ... except when a Friday or Saturday coincides with a day when Muslims habitually fast, such as when it coincides with the day of 'Arafah or the day of 'Ashura, and a person has the habit of fasting on these days, in which case it is not makruh, because a person's habit carries some weight." [Kashshaf al-Qina']
"If there is confusion about the beginning of the month, one should fast for three days, to be sure of fasting on the ninth and tenth days." (Al-Mughni of Ibn Qudamah]
If a person does not know when Muharram began, and he wants to be sure of fasting on the tenth, he should assume that Dhul-Hijjah was thirty days – as is the usual rule – and should fast on the ninth and tenth. Whoever wants to be sure of fasting the ninth as well should fast the eight, ninth and tenth (then if Dhul-Hijjah was twenty-nine days, he can be sure of having fasted Tasu'a and 'Ashura).
But given that fasting on 'Ashura is mustahabb rather than wajib, people are not commanded to look for the crescent of the new moon of Muharram as they are to do in the case of Ramadhan and Shawwal.