What is the ruling on voluntarily breaking a voluntary fast?
General Chapter
Al-Mughni
Book of Fasting
Primary text
It is recommended for the one who begins a voluntary fast to complete it, but it is not obligatory. If one breaks it, there is no need to make it up, according to Ibn Umar and Ibn Abbas, who both broke their fast after starting in the morning. This is the established school of thought (Madhab) of Ahmad, al-Thawri, al-Shafi'i, and Ishaq. The analogy provided is that just as one may cut short a voluntary prayer, one may cut short a voluntary fast.
Supporting text
Al-Nakh'i, Abu Hanifa, and Malik rule that the fast becomes binding upon commencing it and cannot be left except due to an excuse; otherwise, it must be made up. Malik is reported to have said there is no make-up required. This is supported by a report from Aisha (which is disputed) that the Prophet (PBUH) told her and Hafsa, after they broke their voluntary fast with gifted food, to make up the day. This is also supported by analogy to Hajj and Umrah, which become binding upon starting.