The Rules of Forgetfulness (Sahw) in Prayer
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 50
وَ سَأَلَ عُبَيْدُ بْنُ زُرَارَةَ أَبَا عَبْدِ اَللَّهِ عَلَيْهِ اَلسَّلاَمُ: عَنِ اَلرَّجُلِ يُصَلِّي اَلْغَدَاةَ رَكْعَةً وَ يَتَشَهَّدُ وَ يَنْصَرِفُ وَ يَذْهَبُ وَ يَجِيءُ ثُمَّ ذَكَرَ أَنَّهُ إِنَّمَا صَلَّى رَكْعَةً قَالَ «يُضِيفُ إِلَيْهَا رَكْعَةً.
Ubayd ibn Zurarah asked Imam Jafar ibn Muhammad Al-Sadiq عليه السلام about a man who prays the Fajr prayer, offering one rak'ah, then makes the tashahhud (testification), turns away (ends the prayer), and leaves. After some time, he remembers that he only prayed one rak'ah. He asked what should he do. Imam عليه السلام replied: "He should add another rak'ah to it”.
- Ṣaḥīḥ al-Faqīh