Fatwas of the esteemed scholar Shaykh Ibn ‘Uthaymeen

Question: What is the ruling on someone who regularly fasts on Mondays and Thursdays, and one of the days of Tashreeq coincides with those days — should he fast or not?

He replied, may Allah show mercy to him:

If Monday or Thursday coincides with the Days of Tashreeq, then he should not fast them, due to the hadith of ʿĀʾishah and Ibn ʿUmar (may Allah be pleased with them) who said: “No permission was given to fast during the Days of Tashreeq except for one who could not find a sacrificial animal” — meaning the one performing tamattuʿ or qirān during Hajj.

And it is well-known that a prohibition (from Allah or His Messenger) cannot be violated for the sake of performing a Sunnah.