1250/1 - Abu Qatādah (may Allah be pleased with him) reported: The Messenger of Allah (may Allah’s peace and blessings be upon him) was asked about fasting the day of ‘Arafah, and he said: “It expiates the sins of the past year and the coming one.” [Narrated by Muslim]
The past year: It refers to the year preceding the day of ‘Arafah, which occurs during the last month of the Hijri year.
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1) It points out the merit of fasting the day of ‘Arafah for other than pilgrims. It expiates the sins of two years.
2) It shows Allah’s great grace and mercy towards this Ummah, as He gives abundant rewards for simple deeds.
1251/2 - Ibn ‘Abbās (may Allah be pleased with him and his father) reported that the Messenger of Allah (may Allah’s peace and blessings be upon him) fasted on the Day of ‘Āshūrā’ and ordered Muslims to fast it. [Narrated by Al-Bukhāri and Muslim]
1252/3 - Abu Qatādah (may Allah be pleased with him) reported that the Messenger of Allah (may Allah’s peace and blessings be upon him) was asked about fasting the day of ‘Āshūrā’, and he said: “It expiates the sins of the past year.” [Narrated by Muslim]
1253/4 - Ibn ‘Abbās (may Allah be pleased with him and his father) reported that the Prophet (may Allah’s peace and blessings be upon him) said: “If I were to live till the next year, I would definitely fast the ninth day (of Muharram).” [Narrated by Muslim]
1) ‘Āshūrā is the tenth day of Muharram. It is recommended to fast it in gratitude to Allah Almighty, as He saved Prophet Mūsa (Moses) (peace be upon him) and his people and drowned Pharaoh and his people.
2) It is recommended to fast the day of ‘Āshūrā’, as it expiates the sins of the previous year.
The Sunnah is to fast the day of ‘Āshūrā’ along with a day before or a day after, so as to differ from the Jews.