The Rules of Forgetfulness (Sahw) in Prayer
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 50
وَ سَأَلَهُ مَنْصُورُ بْنُ حَازِمٍ: عَنْ رَجُلٍ صَلَّى فَذَكَرَ أَنَّهُ قَدْ زَادَ سَجْدَةً فَقَالَ «لاَ يُعِيدُ صَلاَتَهُ مِنْ سَجْدَةٍ وَ يُعِيدُهَا مِنْ رَكْعَةٍ.
Mansour ibn Hazim asked Imam عليه السلام about a person who prayed and then remembered that he had added an extra prostration. Imam عليه السلام said: "He does not need to repeat his prayer due to the extra prostration, but he must repeat it from the point of the ruku' (bowing)”.
- Ṣaḥīḥ al-Faqīh, p. 46