The Rules of Forgetfulness (Sahw) in Prayer
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 50
وَ رَوَى جَمِيلُ بْنُ دَرَّاجٍ عَنْهُ عَلَيْهِ اَلسَّلاَمُ أَنَّهُ قَالَ: «فِي رَجُلٍ صَلَّى خَمْساً إِنَّهُ إِنْ جَلَسَ فِي اَلرَّابِعَةِ مِقْدَارَ اَلتَّشَهُّدِ فَعِبَادَتُهُ جَائِزَةٌ.
Jamil ibn Darraj narrated from him عليه السلام that He said: "If a man prays five rak'ahs and sits in the fourth rak'ah for the duration of the tashahhud, then his worship is valid."
- Ṣaḥīḥ al-Faqīh