The Rules of Forgetfulness (Sahw) in Prayer
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 50
وَ رُوِيَ عَنْ إِسْحَاقَ بْنِ عَمَّارٍ أَنَّهُ قَالَ: قَالَ لِي أَبُو اَلْحَسَنِ اَلْأَوَّلُ عَلَيْهِ اَلسَّلاَمُ «إِذَا شَكَكْتَ فَابْنِ عَلَى اَلْيَقِينِ» قَالَ قُلْتُ هَذَا أَصْلٌ قَالَ «نَعَمْ.
It is narrated from Ishaq ibn Ammar who said: "Abul Hasan al-Awwal عليه السلام said to me: 'When you are in doubt, rely on what you are certain of.' I said: 'Is this a principle?' Imam عليه السلام said: 'Yes.'
- Ṣaḥīḥ al-Faqīh