The Rules of Forgetfulness (Sahw) in Prayer
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 50
وَ سَأَلَ حَمَّادُ بْنُ عُثْمَانَ أَبَا عَبْدِ اَللَّهِ عَلَيْهِ اَلسَّلاَمُ: عَنْ رَجُلٍ فَاتَهُ شَيْءٌ مِنَ اَلصَّلَوَاتِ فَذَكَرَ عِنْدَ طُلُوعِ اَلشَّمْسِ أَوْ عِنْدَ غُرُوبِهَا قَالَ «فَلْيُصَلِّ حِينَ يَذْكُرُ.
Hammad ibn Uthman asked Imam Jafar ibn Muhammad Al-Sadiq عليه السلام a person who missed part of their prayers and then remembered it at sunrise or sunset. Imam عليه السلام replied: "He should pray as soon as he remembers”.
- Ṣaḥīḥ al-Faqīh, p. 47