Those Muslims who are not participating in Hajj are obligated to observe the Sunnah of fasting on the day of ‘Arafah.

When the Prophet of Allah (peace and blessings of Allah be upon him) was asked about fasting on the day of ‘Arafah, he responded by saying,

It expiates for the past and coming years.

Muslim: 1162

Another report quotes the Prophet Muhammad as saying,

I ask Allah that it may expiate for (the sins of) the year that comes before it and the year that comes after it.

In the Shafi’i book al-Majmu’ (6/428), which is attributed to Al-Nawawi (may Allah have mercy on him), it is written:

In reference to the ruling on this issue, Al-Shafi’i and his companions stated: It is mustahabb to fast on the day of ‘Arafah for the one who is not in ‘Arafah. Because of the hadith of Umm al-Fadl, it is recommended but not mustahabb for the pilgrim who is currently present in ‘Arafah to refrain from fasting, as stated by al-Shafi’i in al-Mukhtasar and his companions in the same text. It was suggested by a number of our companions that he should break his fast on this day. “Among those who declared that it is makruh were al-Darimi, al-Bandaniji, al-Muhamili in al-Majmu’ and al-Musannaf fi’l-Tanbih, in addition to others.

In the book al-Mughni (4/443), which is considered to be a Hanbali text, Ibn Qudamah, may Allah have mercy on him, is quoted as saying:

Today is a day that is both great and noble, as well as a festival that is blessed and full of great virtue. It has been reported from the Prophet (peace and blessings of Allah be upon him) that fasting during this time is equivalent to making atonement for a period of two years.

In the book al-Furoo’ (3/108), which is also considered a Hanbali text, Ibn Muflih, may Allah have mercy on him, is quoted as saying:

According to the general agreement among Islamic scholars, it is mustahabb to observe the fasting during the first nine days of the month of Dhu’l-Hijjah, particularly on the 9^th  day, which is known as the day of Arafat.

In the book Badai’ al-Sanai’ (2/76), which is a Hanafi text, Al-Kasani, may Allah have mercy on him, is quoted as saying:

With regard to fasting on the day of Arafat, it is mustahabb for all Muslims other than those who are performing Hajj. This is because many hadith have recommended fasting on this day, and it is also superior to other days. Because this is combining two acts of worship, it also applies to the pilgrim, provided that it will not make him too weak to stand in ‘Arafah and say dua. But if it will make him too weak, then it is makruh because the virtue of fasting on this day is something that may be made up in some other year, whereas the virtue of standing in Arafat and making dua is something that is not attainable for most people, except once in their lifetime, therefore attaining that virtue (of standing in Arafa and making Prayers) takes precedence. But if it will make him too weak, then he should abstain from it.

It is written in the book Sharh Mukhtasar Khalil by al-Khurashi (6/499), which belongs to the Maliki school of thought, that:

What is meant by the phrase, Fasting on the day of ‘Arafah, for one who is not doing Hajj, and the ten days of Dhu’l-Hijjah, is that fasting on the day of ‘Arafah is mustahabb for the one who is not performing Hajj. As for the pilgrim, it is recommended that he not fast on this day so that he can gain strength for supplicating (Du’a), and the Prophet (peace and blessings of Allah be upon him) did not fast while performing Hajj.

In Hashiyat al-Dasuqi (5/80) it says:

When people say things like “it is recommended to fast on the day of ‘Arafat”, what they really mean is that it is especially recommended on this day, but that fasting is recommended in general.

The question was posed to Shaykh Ibn ‘Uthaymin (may Allah have mercy on him): What is the ruling on fasting on the day of ‘Arafah for non-pilgrims and pilgrims alike?

His response was:

Fasting on the day of ‘Arafat for those who are not making the pilgrimage is a Sunnah. When asked about fasting on the day of ‘Arafat, the Messenger of Allah (peace and blessings of Allah be upon him) responded as follows: “I ask Allah that it may expiate for (the sins of) the year that comes before it and the year that comes after it.” Another report quotes him as saying, “It expiates for the past and forthcoming years.”

Because the Prophet (peace and blessings of Allah be upon him) did not fast on the day of ‘Arafat while performing the Farewell Pilgrimage, it is not a Sunnah for the pilgrim to do so on the day of ‘Arafat. The people were unsure as to whether the Prophet (peace and blessings of Allah be upon him) was fasting on the day of ‘Arafat, so Maymunah (may Allah be pleased with her) sent him some milk while he was standing in ‘Arafat, and he drank it while the people were looking on. This is narrated in Sahih al-Bukhari from Maymunah (may Allah be pleased with her). (Majmu’ Fatawa Ibn ‘Uthaymin, part 20, question no. 404)