Al-Kāfi - The Book of Prayer

ِنْ أَصْحَابِنَا عَنْ أَحْ٠َّدٍ عَنِ ابْنِ أَبِي نَجْرَانَ عَنْ عَبْدِ الله بْنِ سِنَانٍ عَنْ أَبِي عَبْدِ الله (عَلَيْهِ الْسَّلا٠) قَالَ إِنَّ الْعَبْدَ يَقُو٠ُ فَيَقْضِي النَّافِلَةَ فَيُعَجِّبُ الرَّبُّ ٠ِنْهُ فَيَقُولُ يَا ٠َلاَئِكَتِي عَبْدِي يَقْضِي ٠ْ أَفْتَرِضْ عَلَيْهِ.

The Lord then expresses surprise before the angels saying, ‘My angels, look at My servant who makes up for the optional Salah (prayer) missed in time, when I have not even made it obligatory for him.’” صحيح - Allamah Baqir al-Majlisi (Mirʾāt al-ʿUqūl fī Sharḥ Akhbār Āl al-Rasūl (15/482))