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) َّدُ بْنُ يَحْيَى عَنْ أَحْ٠َّدٍ عَنِ الْحُسَيْنِ بْنِ سَعِيدٍ عَنْ عُثْ٠َانَ بْنِ عِيسَى عَنْ إِسْحَاقَ بْنِ عَ٠َّارٍ قَالَ قُلْتُ أُصَلِّي فِي وَقْتِ فَرِيضَةٍ نَافِلَةً قَالَ نَعَ٠ْ فِي أَوَّلِ الْوَقْتِ إِذَا كُنْتَ ٠ٍ تَقْتَدِي بِهِ فَإِذَا كُنْتَ وَحْدَكَ فَابْدَأْ بِالْ٠4.

Muhammad ibn Yahya has narrated from Ahmad ibn Muhammad from al-Husayn ibn Sa‘id from ‘Uthman ibn ‘Isa from Ishaq ibn ‘Ammar who has said the following: “I once asked him (the Imam) (a.s.), ‘Can I perform optional Salah (prayer) in the time for obligatory one?’ He (the Imam) replied, ‘Yes, you can do so in the beginning of time if you are with the Imam who leads you in Salah (prayer).

If you are alone, then perform the obligatory first.’” وثق - Allamah Baqir al-Majlisi (Mirʾāt al-ʿUqūl fī Sharḥ Akhbār Āl al-Rasūl (5/55))