Al-Kāfi

Hadith 1008
worship
4ـ حَمَّادٌ عَنْ حَرِيزٍ عَنْ زُرَارَةَ عَنْ أَبِي جَعْفَرٍ (عَلَيْهِ الْسَّلام) فِي قَوْلِ الله عَزَّ وَجَلَّ إِنَّ الصَّلاةَ كانَتْ عَلَى الْمُؤْمِنِينَ كِتاباً مَوْقُوتاً أَيْ مَوْجُوباً.
4. Hammad has narrated from Hariz from Zurarah from who has said the following: “About the words of Allah, most Majestic, most Glorious, ‘Salah (prayer) in Mawqut (with certain timing) form was written on believers’ abu Ja‘far (a.s.), has said, ‘it means obligatory.’”
Source: Shaykh Muḥammad b. Yaʿqūb al-Kulaynī