السبب الحقيقي لنفس الوجوب هو الإيجاب القديم
The underlying basis of obligation is divine legislation.
People require observable circumstances to know when an obligation becomes applicable. Prayer time is therefore called an outward cause, rather than the ultimate source of legislative authority.
This explains why divine law is valid even before a particular prayer time, although that particular prayer cannot be discharged before its appointed time. The source of the law and the legally specified occasion of its performance are different issues.