The Rules of Forgetfulness (Sahw) in Prayer
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 50
وَ رَوَى سَهْلُ بْنُ اَلْيَسَعِ فِي ذَلِكَ عَنِ اَلرِّضَا عَلَيْهِ اَلسَّلاَمُ أَنَّهُ قَالَ: «يَبْنِي عَلَى يَقِينِهِ وَ يَسْجُدُ سَجْدَتَيِ اَلسَّهْوِ بَعْدَ اَلتَّسْلِيمِ وَ يَتَشَهَّدُ تَشَهُّداً خَفِيفاً.
Sahl ibn al-Yas'a narrated regarding this matter from Imam al-Ridha عليه السلام, who said: "He should base his actions on certainty and perform the two prostrations of forgetfulness after the Tasleem (salutation), and make a brief tashahhud in them."
- Ṣaḥīḥ al-Faqīh