فسبب وجوب الصلاة «الوقت»
The apparent legal cause of the prayer obligation is its time.
The command establishes the duty generally; the prescribed period identifies when an individual prayer is due. A performance before that time does not discharge the later obligation.
For example, someone might pray four voluntary units shortly before noon. When the obligatory noon prayer time arrives, that prior voluntary prayer does not retroactively become the obligatory Zuhr prayer, because its appointed occasion was absent.