Wa `alaykum as-salamu wa rahmatullahi wa barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
It is haram to fast on the Days of Tashreeq, which are the three days after `Eid Al-Adha (11, 12 and 13 Dhul-Hijjah) because the Prophet (peace and blessings be upon him) said: “The Days of Tashreeq are days of eating, drinking, and remembering Allah” (Muslim)
Responding to your question, Sheikh M. S. Al-Munajjid, a prominent Saudi Muslim lecturer and author, states:
First of all, we would like to shed some light on the virtue of the Days of Tashreeq, which are 11, 12, and 13 Dhul-Hijjah. There are several verses and hadiths that speak of their virtue:
Allah says, “And remember Allah during the appointed Days.” (Al-Baqarah 2:203)
These are the Days of Tashreeq. This was the view of Ibn `Umar and most of the scholars.
The Prophet (peace and blessings be upon him) said concerning the Days of Tashreeq: “They are days of eating, drinking, and remembering Allah.” Dhikr (remembering Allah) is enjoined during the Days of Tashreeq in several ways:
-Remembering Allah immediately after the prescribed Prayers by reciting the Takbeer that is recited before the `Eid Prayer. This is prescribed until the end of the Days of Tashreeq, according to the majority of scholars.
-Remembering Him by saying Bismillah, Allahu Akbar when slaughtering the sacrificial animal.
-Remembering Allah when eating and drinking. It is prescribed when eating and drinking to say bismillah at the beginning, and to praise Him with al-hamdu lillah at the end. According to the hadith narrated from the Prophet (peace and blessings be upon him): “Allah likes His slave when he eats something to praise Him for it, and when he drinks something to praise Him for it.” (Muslim)
-Remembering Him by saying Allahu Akbar when throwing the stones at the Jamarat on the Days of Tashreeq. This applies only to the pilgrims on Hajj.
-Remembering Allah in general. It is mustahabb (recommended) to make a lot of dhikr during the Days of Tashreeq. `Umar (may Allah be pleased with him) used to recite Takbeer of `Eid in his tent in Mina, and when the people heard him, they recited it, too, and Mina echoed with the sound of their Takbeer. Allah says: “So when you have accomplished your manasik [rituals of Hajj], remember Allah as you remember your forefathers or with far more remembrance. But of mankind there are some who say: ‘Our Lord! Give us (Your Bounties) in this world!’ and for such there will be no portion in the Hereafter. And of them there are some who say: ‘Our Lord! Give us in this world that which is good and in the Hereafter that which is good, and save us from the torment of the Fire!’.” (Al-Baqarah 2:200-201)
Fasting on the two days of `Eid is haram (forbidden), as indicated by the hadith of Abu Sa`id Al-Khudri (may Allah be pleased with him) who said: The Prophet (peace and blessings be upon him) forbade fasting on the day of Al-Fitr and [the day of] An-Nahr [Sacrifice].” (Al-Bukhari and Muslim)
The scholars are unanimously agreed that fasting on these two days is haram.
It is also haram to fast on the Days of Tashreeq, which are the three days after `Eid Al-Adha (11, 12 and 13 Dhul-Hijjah) because the Prophet (peace and blessings be upon him) said: “The Days of Tashreeq are days of eating, drinking, and remembering Allah” (Muslim)
Abu Dawud narrated from Abu Murrah, the freed slave of Umm Hani, that he entered with `Abdullah ibn `Amr upon his father, `Amr ibn Al-`Aas. He offered them food and said, “Eat.” He [Abu Murrah] said, “I am fasting.” `Amr said, “Eat, for these are the days on which the Messenger of Allah (peace and blessings be upon him) would tell us not to fast and forbade us to fast, and these are the Days of Tashreeq.” (Abu Dawud)
But it is permissible for a pilgrim who does not have a hadi (sacrificial animal) to fast on the Days of Tashreeq. It was narrated that `A’ishah and Ibn `Umar (may Allah be pleased with them) said: No one was allowed to fast on the Days of Tashreeq except one who could not find a hadi. (Al-Bukhari)
The late Sheikh Ibn `Uthaymeen, a prominent Saudi Muslim scholar, (may Allah have mercy on his soul) adds,
It is permissible for those who are doing Hajj qiran or tamattu`, if they cannot find a hadi, to fast these three days so that the Hajj season will not end before they fast. But apart from that, it is not permissible to fast on these days, even if a person is obliged to fast for two consecutive months; he should not fast on the day of `Eid and the three days following it, but then he should continue his fast.
Allah Almighty knows best.
Excerpted, with slight modifications from, www.islamqa.info.