Making up the Night Prayer
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 76
وَ رَوَى مُحَمَّدُ بْنُ مُسْلِمٍ عَنْ أَبِي جَعْفَرٍ عَلَيْهِ اَلسَّلاَمُ قَالَ: قُلْتُ لَهُ رَجُلٌ مَرِضَ فَتَرَكَ اَلنَّافِلَةَ فَقَالَ «يَا مُحَمَّدُ لَيْسَتْ بِفَرِيضَةٍ إِنْ قَضَاهَا فَهُوَ خَيْرٌ يَفْعَلُهُ وَ إِنْ لَمْ يَفْعَلْ فَلاَ شَيْءَ عَلَيْهِ.
Muhammad ibn Muslim narrates that Imam Muhammad ibn Ali Al-Baqir عليه السلام said: "I asked him about a man who became ill and thus did not perform the supererogatory (nafilah) prayers. Imam عليه السلام said: 'O’ Muhammad, they are not obligatory. If he makes them up, it is a good deed he performs, and if he does not, there is no obligation upon him.'"
- Ṣaḥīḥ al-Faqīh, p. 71