Section III: The Ruling of Completing a Day Intended to Fast

Firstly: Ruling of completing a wajib day of fasting
If a person starts fasting a wajib fast, like making up a day, for expiation, or the like, then he must complete it and cannot break it except for a legally valid reason. This is by agreement of the four schools of jurisprudence.

Secondly: Ruling of completing a voluntary day of fasting
It is mustahabb to complete a voluntary fast if it has been started, but it is not wajib. This is the position of the Shafi`is, Hanbalis, a group of the Salaf, and the choice of Ibn `Uthaymin.

Thirdly: Ruling of making up voluntary fasts if broken
If a person breaks a voluntary fast, then they do not have to make it up. This is the position of the Shafi`is, Hanbalis, and a group of the Salaf.