The Rules of Forgetfulness (Sahw) in Prayer
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 50
وَ أَمَّا حَدِيثُ صَفْوَانَ بْنِ مِهْرَانَ اَلْجَمَّالِ عَنْ أَبِي عَبْدِ اَللَّهِ عَلَيْهِ اَلسَّلاَمُ قَالَ: وَ سَأَلْتُهُ عَنْ سَجْدَتَيِ اَلسَّهْوِ فَقَالَ «إِذَا نَقَصْتَ فَقَبْلَ اَلتَّسْلِيمِ وَ إِذَا زِدْتَ فَبَعْدَهُ.
As for the narration of Safwan ibn Mehran al-Jammal from Imam Jafar ibn Muhammad Al-Sadiq عليه السلام that He said: "I asked him about the two prostrations of forgetfulness (Sajdat al-Sahw), Imam عليه السلام said: 'If you have made a mistake by reducing something in your prayer, then [perform the prostrations] before the Tasleem. But if you have added something, then [perform the prostrations] after the Tasleem.'"
- Ṣaḥīḥ al-Faqīh