Chapter on the Rulings on Forgetfulness (Ahkam al-Sahw)
Tahdhīb al-Aḥkām|Volume 2|Book 1|Chapter 16
فَأَمَّا مَا رَوَاهُ اَلْحُسَيْنُ بْنُ سَعِيدٍ عَنْ صَفْوَانَ عَنِ اَلْعَلاَءِ عَنْ مُحَمَّدٍ عَنْ أَبِي جَعْفَرٍ عَلَيْهِ اَلسَّلاَمُ قَالَ: سُئِلَ عَنْ رَجُلٍ دَخَلَ مَعَ اَلْإِمَامِ فِي صَلاَتِهِ وَ قَدْ سَبَقَهُ بِرَكْعَةٍ فَلَمَّا فَرَغَ اَلْإِمَامُ خَرَجَ مَعَ اَلنَّاسِ ثُمَّ ذَكَرَ بَعْدَ ذَلِكَ أَنَّهُ فَاتَتْهُ رَكْعَةٌ قَالَ« يُعِيدُهَا رَكْعَةً وَاحِدَةً»
As for what was narrated by al-Husayn ibn Sa‘id from Safwan by al-‘Ala’ from Muhammad by Abu Ja‘far عليه السلام, peace be upon him, who said:
Imam عليه السلام was asked about a man who joined the Imam (of prayer) in prayer while the Imam (of prayer) had already performed one rak‘ah before him. When the Imam completed the prayer, the man left with the people, then afterward remembered that he had missed a rak‘ah. Imam عليه السلام said: "He should repeat it as one rak‘ah."
- Tahdhīb al-Aḥkām fī Sharḥ al-Muqniʿah, vol. 2, p. 372
- Ṣaḥīḥ al-Tahdhīb