The Rules of Forgetfulness (Sahw) in Prayer
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 50
وَ قَدْ رُوِيَ عَنِ اَلصَّادِقِ عَلَيْهِ اَلسَّلاَمُ أَنَّهُ قَالَ: «اَلْإِنْسَانُ لاَ يَنْسَى تَكْبِيرَةَ اَلاِفْتِتَاحِ.
It has been narrated from Imam Jafar ibn Muhammad Al-Sadiq عليه السلام that He said: "A person does not forget the Takbira al-Ihram (the opening Takbir)."
- Ṣaḥīḥ al-Faqīh