Is it forbidden to fast the days of Tashriq (the three days following Eid al-Adha)?
General Chapter
Al-Mughni
Book of Fasting
Primary text
Fasting the days of Tashriq is also forbidden based on the report that they are 'days of eating and drinking, and remembrance of Allah the Almighty.' This is the position of the majority of scholars for voluntary fasting. The apparent understanding is that those who fasted them were not informed of the Prophet's prohibition, or they would not have done otherwise.
Supporting text
There is a difference of opinion regarding fasting them as a required fast (Fard). One view is that it is impermissible as it is forbidden, similar to the Eid days. The second view is that it is valid for the obligatory fast, based on a sound report from Ibn Umar and Aisha allowing it for the one who cannot afford the sacrificial animal (Hady), which is then analogized to every obligatory fast.