Volume 3, Book 31, Number 116:
Narrated Ibn ‘Umar (radiallaahu `anhu):
The Prophet (sallallaahu `alayhi wasallam) observed the fast on the 10th of Muharram (‘Ashura), and ordered (Muslims) to fast on that day, but when the fasting of the month of Ramadan was prescribed, the fasting of the ‘Ashura’ was abandoned. ‘Abdullah (radiallaahu `anhu) did not use to fast on that day unless it coincided with his routine fasting by chance.
Volume 3, Book 31, Number 117:
Narrated ‘Aisha (radiallaahu `anhu):
(The tribe of) Quraish used to fast on the day of Ashura’ in the Pre-lslamic period, and then Allah’s Apostle (sallallaahu `alayhi wasallam) ordered (Muslims) to fast on it till the fasting in the month of Ramadan was prescribed; whereupon the Prophet (sallallaahu `alayhi wasallam) said, “He who wants to fast (on ‘Ashura’) may fast, and he who does not want to fast may not fast.”
Ibn ‘Abbas (may Allah be pleased with him) said: “The Prophet (peace and blessings of Allah be upon him) came to Madinah and saw the Jews fasting on the day of ‘Ashura’. He said, ‘What is this?’ They said, ‘This is a righteous day, it is the day when Allah saved the Children of Israel from their enemies, so Musa fasted on this day.’ He said, ‘We have more right to Musa than you,’ so he fasted on that day and commanded [the Muslims] to fast on that day.” [Reported by al-Bukhari, 1865] “This is a righteous day” – in a report narrated by Muslim, [the Jews said:] “This is a great day, on which Allah saved Musa and his people, and drowned Pharaoh and his people.” “Musa fasted on this day” – a report narrated by Muslim adds: ” … in thanksgiving to Allah, so we fast on this day.” According to a report narrated by al-Bukhari: ” … so we fast on this day to venerate it.” A version narrated by Imam Ahmad adds: “This is the day on which the Ark settled on Mount Judi, so Nuh fasted this day in thanksgiving,” and “commanded [the Muslims] to fast on that day” – according to another report also narrated by al-Bukhari: “He said to his Companions: ‘You have more right to Musa than they do, so fast on that day.’ ”
The practice of fasting on ‘Ashura’ was known even in the days of Jahiliyyah, before the Prophet’s mission. It was reported that ‘A’ishah (may Allah be pleased with her) said: “The people of Jahiliyyah used to fast on that day … ”
Al-Qurtubi said: “Perhaps Quraysh used to fast on that day on the basis of some past law, such as that of Ibrahim, upon whom be peace.”
It was also reported that the Prophet (peace and blessings of Allah be upon him) used to fast on ‘Ashura’ in Makkah, before he migrated to Madinah. When he migrated to Madinah, he found the Jews celebrating this day, so he asked them why, and they replied as described in the hadith quoted above. He commanded the Muslims to be different from the Jews, who took it as a festival, as was reported in the hadithof Abu Musa (may Allah be pleased with him), who said: “The Jews used to take the day of ‘Ashura’ as a festival [according to a report narrated by Muslim: the day of ‘Ashura’ was venerated by the Jews, who took it as a festival. According to another report also narrated by Muslim: the people of Khaybar (the Jews) used to take it as a festival and their women would wear their jewellery and symbols on that day].The Prophet (peace and blessings of Allah be upon him) said: ‘So you [Muslims] should fast on that day.’ ” [Reported by al-Bukhari] Apparently the motive for commanding the Muslims to fast on this day was the desire to be different from the Jews, so that the Muslims would fast when the Jews did not, because people do not fast on a day of celebration. [Summarized from the words of al-Hafidh Ibn Hajar – may Allah have mercy on him – in Fath al-Bari Sharh ‘ala Sahih al-Bukhari]
Fasting on ‘Ashura’ was a gradual step in the process of introducing fasting as a prescribed obligation in Islam. Fasting appeared in three forms. When the Messenger of Allah (peace and blessings of Allah be upon him) came to Madinah, he told the Muslims to fast on three days of every month and on the day of ‘Ashura’, then Allah made fasting obligatory when He said (interpretation of the meaning): ” … observing the fasting is prescribed for you … ” [Al-Qur’an 2:183] [Ahkam al-Qur’an by al-Jassas, part 1]
The obligation was transferred from the fast of ‘Ashura’ to the fast of Ramadhan, and this one of the proofs in the field of Usul al-Fiqh that it is possible to abrogate a lighter duty in favour of a heavier duty.
Before the obligation of fasting ‘Ashura’ was abrogated, fasting on this day was obligatory, as can be seen from the clear command to observe this fast. Then it was further confirmed later on, then reaffirmed by making it a general command addressed to everybody, and once again by instructing mothers not to breastfeed their infants during this fast. It was reported from Ibn Mas’ud that when fasting Ramadhan was made obligatory, the obligation to fast ‘Ashura’ was lifted, i.e., it was no longer obligatory to fast on this day, but it is still desirable (mustahabb).
[Taken from SunnahOnline.com]