Is the fast valid if one intends to break the voluntary fast but does not renew the intention to fast afterward?
General Chapter
Al-Mughni
Book of Fasting
Primary text
If one intends to break a voluntary fast (*nawafil*) and subsequently does not renew the intention to fast, the fast is invalid because the initial intention is severed, and no other intention exists, resembling one who never intended to fast at all. If the person subsequently renews the intention to fast, the fast becomes valid, similar to one who wakes up without intending to fast. This is because the intention to break the fast only invalidates the obligatory fast due to the required continuity of intention throughout the day. Voluntary fasts differ in this regard, so the intention to break the fast during a time when the intention to fast is not strictly required throughout does not prevent validity. The intention to break the fast is no greater than the absence of intention at that time, and absence of intention does not prevent validity if one intends afterward. This is unlike the obligatory fast, which is not validated by an intention made during the daylight hours.
Supporting text
It is narrated from Ahmad that if one begins the day fasting an obligatory fast and resolves to break it, but does not break it and then decides to complete the fast, it is not sufficient unless the intention to fast covers the entire day. He stated that this matter is easier for voluntary fasts. The apparent meaning of this supports the primary ruling. The validity of the voluntary fast is demonstrated by the Prophet (peace be upon him) asking his household if they had food; if they said no, he would state, "Then I am fasting today."