Does apostasy (Riddah) during fasting invalidate the fast?
General Chapter
Al-Mughni
Book of Fasting
Primary text
There is no known difference of opinion among scholars that apostasy from Islam during the fast invalidates that day's fast, and the person must make it up upon returning to Islam. This applies whether the apostasy occurred during the day or after it concluded, and whether it was through believing in a matter of disbelief, doubting something that necessitates disbelief, or uttering a word of disbelief, whether in jest or seriously. This is because fasting is an act of worship conditioned upon intention, which apostasy nullifies, similar to prayer and Hajj. Furthermore, it is a pure act of worship negated by disbelief, like prayer. This is the position of Al-Shafi'i, Abu Thawr, and the Ashab al-Ra'y.
Supporting text
The Ashab al-Ra'y stipulated that if the apostate returns and forms the intention (to fast) before midday, it suffices, based on their principle that fasting can be valid with an intention formed during the day. Ibn Hamid held that the fast is not invalidated because it is an obligation requiring continuation, and thus should not be invalidated by the intention to exit it, similar to Hajj.