The Rules of Forgetfulness (Sahw) in Prayer
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 50
وَ قَدْ رُوِيَ: «أَنَّهُ يُصَلِّي رَكْعَةً مِنْ قِيَامٍ وَ رَكْعَتَيْنِ مِنْ جُلُوسٍ.
It has been narrated: "He should pray one rak'ah standing and two rak'ahs sitting."
- Ṣaḥīḥ al-Faqīh