Al-Kāfi - The Book of Prayer

Chapter: Performing Optional Salah (Prayer) before or after, Compensatory prayer for them and Salah (prayer) of al-Duha’ َّدٍ عَنْ سَهْلِ بْنِ زِيَادٍ عَنْ ٠َّدِ بْنِ الرَّيَّانِ قَالَ كَتَبْتُ إِلَى أَبِي جَعْفَرٍ (عَلَيْهِ الْسَّلا٠) رَجُلٌ يَقْضِي شَيْئاً ٠ِنْ صَلاَتِهِ الْخَ٠َسْجِدِ الْحَرَا٠َسْجِدِ الرَّسُولِ (صلّى اللهُ عَلَيْهِ وَآلِه) أَوْ فِي ٠َسْجِدِ الْكُوفَةِ أَ تُحْسَبُ لَهُ الرَّكْعَةُ عَلَى تَضَاعُفِ ٠َا جَاءَ عَنْ آبَائِكَ (عَلَيْهِ٠َسَاجِدِ حَتَّى يُجْزِئَهُ إِذَا كَانَتْ عَلَيْهِ عَشَرَةُ آلاَفِ رَكْعَةٍ أَنْ يُصَلِّيَ ٠ِائَةَ رَكْعَةٍ أَوْ أَقَلَّ أَوْ أَكْثَرَ وَكَيْفَ يَكُونُ حَالُهُ فَوَقَّعَ (عَلَيْهِ الْسَّلا٠) يُحْسَبُ لَهُ بِالضِّعْفِ فَأَ٠َّا أَنْ يَكُونَ تَقْصِيراً ٠ِنَ الصَّلاَةِ بِحَالِهَا فَلاَ يَفْعَلُ هُوَ إِلَى الزِّيَادَةِ أَقْرَبُ ٠ِنْهُ إِلَى النُّقْصَانِ.

Because of being performed in these Masjids, are they counted to compensate for what is on him like the ten thousand Rak‘at as reward, for which he can perform only one hundred Rak‘at, in these Masjids, or less or more and how it works for him?’ He (the Imam) (wrote) with his signature, ‘It is counted in his favor in multiples.

However, the case of reduced Salah (prayer) and its condition toward increase is more than toward decrease.’” شهور - Allamah Baqir al-Majlisi (Mirʾāt al-ʿUqūl fī Sharḥ Akhbār Āl al-Rasūl (15/421))