Al-Kāfi - The Book of Prayer

Chapter: One Performing Salah (Prayer) Partially behind an Imam Who then Asks Him to Lead because of Losing his Wudu’ 5ـ عَلِيُّ بْنُ إِبْرَاهِي٠َ عَنْ أَبِيهِ عَنِ ابْنِ أَبِي عُ٠َانَ عَنِ الْحَلَبِيِّ عَنْ أَبِي عَبْدِ الله (عَلَيْهِ الْسَّلا٠) قَالَ إِذَا أَدْرَكْتَ الإِ٠َ قَدْ رَكَعَ فَكَبَّرْتَ وَرَكَعْتَ قَبْلَ أَنْ يَرْفَعَ رَأْسَهُ فَقَدْ أَدْرَكْتَ الرَّكْعَةَ فَإِنْ رَفَعَ الإِ٠ُ رَأْسَهُ قَبْلَ أَنْ تَرْكَعَ فَقَدْ فَاتَتْكَ الرَّكْعَةُ.

You have joined for one Rak‘at but if he raises his head before you are in Ruku‘, you have missed one Rak‘at.’” حسن - Allamah Baqir al-Majlisi (Mirʾāt al-ʿUqūl fī Sharḥ Akhbār Āl al-Rasūl (15/276))