Fiqh-us-Sunnah Volume 3, Zakaat and Fasting, Fiqh 3.123.

It is forbidden to do so because there are certain days of the year on which one is not allowed to fast. The Messenger of Allah said: “There is no [reward for] fasting for the one who perpetually fasts.” This is related by Ahrnad, al-Bukhari, and Muslim.

If one breaks his fast during the days of ‘id and the days of tashreeq, then his perpetual fasting would no longer be considered disliked. In his comments on this issue, at-Tirmizhi says: “A group of scholars dislike fasting every day if it includes the ‘ids [‘id al-Fitr, ‘id al-Azha] and the days of tashreeq. If one breaks his fast on those days, his action is no longer disliked, as he is no longer fasting the whole year.” The scholars are Malik, ash-Shaf’i, Ahrnad, and Ishaq.

The Prophet approved of Hamzah al-Aslami’s numerous fasts when he told him: “Fast if you wish and break your fast if you wish.” This hadith was mentioned earlier.