The Chapter on Obligatory Prayers
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 30
وَقَالَ زُرَارَةُ بْنُ أَعْيَنَ قَالَ أَبُو جَعْفَرٍ عَلَيْهِ اَلسَّلاَمُ: «كَانَ اَلَّذِي فَرَضَ اَللَّهُ عَزَّ وَ جَلَّ عَلَى اَلْعِبَادِ عَشْرَ رَكَعَاتٍ وَ فِيهِنَّ اَلْقِرَاءَةُ وَ لَيْسَ فِيهِنَّ وَهْمٌ يَعْنِي سَهْوٌ فَزَادَ رَسُولُ اَللَّهِ صَلَّى اَللَّهُ عَلَيْهِ وَ آلِهِ سَبْعاً وَ فِيهِنَّ اَلسَّهْوُ وَ لَيْسَ فِيهِنَّ اَلْقِرَاءَةُ فَمَنْ شَكَّ فِي اَلْأَوَّلَتَيْنِ أَعَادَ حَتَّى يَحْفَظَ وَ يَكُونَ عَلَى يَقِينٍ وَ مَنْ شَكَّ فِي اَلْأَخِيرَتَيْنِ عَمِلَ بِالْوَهْمِ.
Zurarah ibn A'yan narrated that Imam Muhammad ibn Ali Al-Baqir عليه السلام said: "What Allah سبحانه وتعالى, the Mighty and Majestic, initially made obligatory upon His servants were ten rak'ahs (units of prayer), in which there is recitation and no room for doubt (meaning no room for forgetfulness or error). The Messenger of Allah صلى الله عليه وعلى آله وسلم added seven rak'ahs, in which forgetfulness may occur but there is no obligatory recitation. So, whoever doubts during the first two (rak'ahs) must repeat (the prayer) until they are certain. However, if one doubts in the last two (rak'ahs), they should act according to their assumption."
- Ṣaḥīḥ al-Faqīh, p. 33