The Rules of Forgetfulness (Sahw) in Prayer
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 50
وَ رَوَى زُرَارَةُ عَنْ أَبِي جَعْفَرٍ عَلَيْهِ اَلسَّلاَمُ أَنَّهُ قَالَ: «إِذَا أَنْتَ كَبَّرْتَ فِي أَوَّلِ صَلاَتِكَ بَعْدَ اَلاِسْتِفْتَاحِ بِإِحْدَى وَ عِشْرِينَ تَكْبِيرَةً ثُمَّ نَسِيتَ اَلتَّكْبِيرَ كُلَّهُ أَوْ لَمْ تُكَبِّرْهُ أَجْزَأَكَ اَلتَّكْبِيرُ اَلْأَوَّلُ عَنْ تَكْبِيرَةِ اَلصَّلاَةِ كُلِّهَا.
Zurarah narrated from Imam Muhammad ibn Ali Al-Baqir عليه السلام that He said: "When you say the Takbir at the beginning of your prayer after the opening Takbir (Takbir al-Ihram), if you say twenty-one Takbirs and then forget the remaining Takbirs, or fail to say them, the first Takbir will suffice for all the Takbirs in the prayer”.
- Ṣaḥīḥ al-Faqīh