قال تعالي 
إِنَّ الصَّلاَةَ كَانَتْ عَلَى الْمُؤْمِنِينَ كِتَابًا مَّوْقُوتًا 

Verily the prayer is enjoined on the believers at fixed hours.
Surah Al Nisa (Chapter 4: Verse 103)