The Ramaḍān fast is valid from one who is sound of body and resident, with an unrestricted intention, an intention of supererogation (nafl), or an intention of another obligation.
As for a specified vow (nadhr), it is valid with an unrestricted intention or an intention of supererogation—not with an intention of another obligation.
If the sick person or traveller intends Ramaḍān as another obligation, it is valid; if he observes it voluntarily (nafl), there are two narrations.
Ruling448
والنذرAnd [regarding] the vowContextual glossالمطلق،unrestricted,Contextual glossوالكفّارة،and expiation,Contextual glossوقضاءand making upContextual glossرمضان،Ramaḍān,Contextual glossونحوها،and their likes,Contextual glossلاnotContextual glossيصحّis validContextual glossبنيّةwith intentionContextual glossفيduringContextual glossالنهار.the day.Contextual gloss
An unrestricted vow, expiation (kaffārah), making up Ramaḍān, and the like do not become valid by an intention formed during daytime.
Ruling449
ويستحبّAnd it is recommendedContextual glossطلبseekingContextual glossالهلالthe new moonContextual glossليلةnightContextual glossثلاثينthirtyContextual glossمنofContextual glossشعبانShaʿbānContextual glossورمضان،and Ramaḍān,Contextual glossفإنSo ifContextual glossلمdid notContextual glossيرseeContextual glossفلاthen neitherContextual glossصومfastContextual glossولاnorContextual glossفطر.breaking fast.Contextual gloss
Seeking sighting of the new moon on the night preceding the thirtieth of Shaʿbān and of Ramaḍān is recommended; if it is not seen, neither fasting nor breaking the fast [that evening] applies.
Whoever sights the crescent alone and his testimony is rejected should fast; if he breaks his fast after the rejection, make-up alone is obligatory on him—nothing further—likewise if he breaks before it according to some; if he completes thirty days he should not break the fast alone; if he breaks, there is no expiation upon him.
Regarding Ramaḍān’s crescent under cloud cover, the testimony of one upright witness is accepted—even if he is a slave, a woman, or one punished for calumny (qadhf); if they fast thirty days yet do not sight the crescent, breaking the fast is differed over—unlike with two witnesses.
If they complete Shaʿbān then fast Ramaḍān yet it amounts to twenty-eight days: if they counted Shaʿbān by sighting its crescent they make up one day; otherwise they make up two.