Abu Qatadah reported
that the Messenger of Allah said:

"Fasting on the day of 'Arafah is an expiation for two years,
the year preceding it and the year following it.
Fasting the day of 'Ashurah is an expiation for the year preceding it."

This is related by "the group," except for al-Bukhari and at-Tirmidhi.

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Abu Hurairah stated:

"The Messenger of Allah
forbade fasting on the day of 'Arafah for one who is actually at 'Arafah."

This is related by Ahmad, Abu Dawud, an-Nasa'i, and Ibn Majah.

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At-Tirmidhi comments:

"The scholars prefer that
the day of 'Arafah be fasted unless one is actually at 'Arafah."