The Rules of Forgetfulness (Sahw) in Prayer
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 50
وَ سَأَلَ أَحْمَدُ بْنُ مُحَمَّدِ بْنِ أَبِي نَصْرٍ اَلْبَزَنْطِيُّ اَلرِّضَا عَلَيْهِ اَلسَّلاَمُ: عَنْ رَجُلٍ نَسِيَ أَنْ يُكَبِّرَ تَكْبِيرَةَ اَلاِفْتِتَاحِ حَتَّى كَبَّرَ لِلرُّكُوعِ فَقَالَ «أَجْزَأَهُ.
Ahmad ibn Muhammad ibn Abi Nasr al-Bazanti asked Imam al-Ridha عليه السلام about a man who forgot to say the Takbir al-Ihram (opening Takbir) until he had already said the Takbir for bowing (ruku'). Imam عليه السلام replied: "It suffices him”.
- Ṣaḥīḥ al-Faqīh