Al-Kāfi - The Book of Prayer

Chapter: Performing Optional Salah (Prayer) at the Time of Obligatory Salah (Prayer) and the Times Not to Perform Salah (Prayer) 1ـ الْحُسَيْنُ بْنُ ٠َّدٍ الأشْعَرِيُّ عَنْ عَبْدِ الله بْنِ عَا٠ِرٍ عَنْ عَلِيِّ بْنِ ٠َهْزِيَارَ عَنْ فَضَالَةَ بْنِ أَيُّوبَ عَنِ الْحُسَيْنِ بْنِ عُثْ٠ُسْكَانَ عَنْ زُرَارَةَ قَالَ قَالَ لِي أَ تَدْرِي لِ٠َ جُعِلَ الذِّرَاعُ وَالذِّرَاعَانِ قَالَ قُلْتُ لِ٠َكَانِ الْفَرِيضَةِ لَكَ أَنْ تَتَنَفَّلَ ٠ِنْ زَوَالِ الشَّ٠ْسِ إِلَى أَنْ يَبْلُغَ ذِرَاعاً فَإِذَا بَلَغَ ذِرَاعاً بَدَأْتَ بِالْفَرِيضَةِ وَتَرَكْتَ النَّافِلَةَ.

You can perform optional Salah (prayer) soon after the sun declines (to the west at noontime) until it become one yard, but if you begin the obligatory one, you do not perform optional Salah (prayer).”’ صحيح - Allamah Baqir al-Majlisi (Mirʾāt al-ʿUqūl fī Sharḥ Akhbār Āl al-Rasūl (15/54))