Especially the tenth day, ‘Ashura’
“I [Ibn ‘Abbas (ra)] never saw the Prophet [saws] seeking to observe saum on a day more than this day the day of ‘Ashura’ or this month the month of Ramadan.”[30]
“One of the wives of the Prophet [saws] said, ‘The Prophet [saws] used to fast on the day of ‘Ashura’, nine days of Dhul-Hijjah and three days of each month: the first Monday of the month and two Thursdays.’”[31]
“The Messenger of Allah [saws] used to fast [the first] nine days of Dhul-Hijjah and the day of ‘Ashura’ and three days every month: the first Monday of the month and Thursday.”[32]
“There are four things which the Prophet [saws] never gave up: fasting ‘Ashura’, [fasting during] the ten days, [fasting] three days of each month and praying two raka’ah before Al-Ghadah [Fajr].”[33]
The reward for fasting ‘Ashura’
“Fasting the day of ‘Ashura’ I hope will expiate for the sins of the previous year.”[34]
“A man came to the Prophet [saws] and said, ‘O Messenger of Allah! How do you fast?’ The Messenger of Allah [saws] got angry at what he said. When ‘Umar [ra] observed this he said, “We are satisfied with Allah [swt] as our Lord Islam as our religion and Muhammad [saws] as our Prophet. We seek refuge in Allah [swt] from the anger of Allah [swt] and the anger of His Messenger.’ Umar [ra] kept on repeating these words until the anger of the Prophet [saws] receded. He then said, ‘O Messenger of Allah [saws]! What about a person who fasts continuously?’ He said, ‘He is neither fasting nor breaking fast.’ He said, ‘O Messenger of Allah [saws]! What about a person who fasts two days and does not fast one day?’ He said, ‘Is there anybody who can do that?’ He said, ‘O Messenger of Allah [saws]! What about a person who fasts for a day and breaks his fast for a day?’ He said, ‘That is the fast of Dawud [as].’ He said, ‘O Messenger of Allah [saws]! What about a person who fasts for a day and breaks his fast for two days?’ He said, ‘I wish I could do that.’ Then the Messenger of Allah [saws] said, ‘[Fasting] three days from every month and Ramadan to Ramadan this is fasting all the time. And fasting the day of ‘Arafah; I hope from Allah [swt] that it may expiate for [the sins of] the preceding and the following year. And fasting the day of ‘Ashura’; I hope from Allah [swt] that it may atone for the sins of the preceding year.’”[35]
Previously obligatory
“The Messenger of Allah [saws] used to fast ‘Ashura’ and he ordered [others] to fast it too.”[36]
“The people of Khaibar used to fast on the day of ‘Ashura’ and they took it as a festival and dressed their women in their jewellery and finery on that day. The Messenger of Allah [saws] said, ‘Then you [Muslims] should fast [on that day].’”[37]