فَإِذَا قَضَيْتُمُ الصَّلاَةَ فَاذْكُرُواْ اللّهَ قِيَامًا وَقُعُودًا وَعَلَى جُنُوبِكُمْ فَإِذَا اطْمَأْنَنتُمْ فَأَقِيمُواْ الصَّلاَةَ إِنَّ الصَّلاَةَ كَانَتْ عَلَى الْمُؤْمِنِينَ كِتَابًا مَّوْقُوتًا
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سورة النساء
(4/103) And when you have completed the prayer, remember Allah standing, sitting, or lying (down) on your sides; but when you are in safety, keep up the prayer in congregation; surely the prayer is enjoined on the believers at fixed hours as an obligation.
Surah An-Nisa