The Rules of Forgetfulness (Sahw) in Prayer
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 50
وَ رَوَى زُرَارَةُ عَنْ أَحَدِهِمَا عَلَيْهِمَا اَلسَّلاَمُ قَالَ: «إِنَّ اَللَّهَ تَبَارَكَ وتَعَالَى فَرَضَ اَلرُّكُوعَ وَ اَلسُّجُودَ وَ اَلْقِرَاءَةُ سُنَّةٌ فَمَنْ تَرَكَ اَلْقِرَاءَةَ مُتَعَمِّداً أَعَادَ اَلصَّلاَةَ وَ مَنْ نَسِيَ فَلاَ شَيْءَ عَلَيْهِ.
Zurarah narrated from one of the Imams that He said: "Indeed, Allah سبحانه وتعالى, Blessed and Exalted, has made ruku' (bowing) and Sujood (prostration) obligatory, while recitation is a Sunnah. Whoever intentionally leaves out the recitation must repeat the prayer, but if someone forgets, there is no fault upon them."
- Ṣaḥīḥ al-Faqīh, p. 46