The Rules of Forgetfulness (Sahw) in Prayer
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 50
وَ فِي رِوَايَةِ اِبْنِ أَبِي عُمَيْرٍ عَنْ مُحَمَّدِ بْنِ أَبِي حَمْزَةَ أَنَّ اَلصَّادِقَ عَلَيْهِ اَلسَّلاَمُ قَالَ: «إِذَا كَانَ اَلرَّجُلُ مِمَّنْ يَسْهُو فِي كُلِّ ثَلاَثٍ فَهُوَ مِمَّنْ كَثُرَ عَلَيْهِ اَلسَّهْوُ.
In the narration of Ibn Abi Umayr, from Muhammad ibn Abi Hamza said: "The Imam Jafar ibn Muhammad Al-Sadiq عليه السلام said: 'If a man is one who forgets or becomes distracted in every third part of his prayer, then he is among those for whom distractions are frequent.'"
- Ṣaḥīḥ al-Faqīh