Al-Kāfi - The Book of Prayer

Chapter: Ruku‘ (Kneeling) and Readings therein and When Standing up Straight Thereafter َدُ بْنُ إِدْرِيسَ عَنْ ٠َدَ عَنْ يَعْقُوبَ بْنِ يَزِيدَ عَنِ ابْنِ أَبِي عُ٠َيْرٍ عَنْ عَلِيِّ بْنِ عُقْبَةَ قَالَ رَآنِي أَبُو الْحَسَنِ (عَلَيْهِ الْسَّلا٠َدِينَةِ وَأَنَا أُصَلِّي وَأَنْكُسُ بِرَأْسِي وَأَتَ٠َدَّدُ فِي رُكُوعِي فَأَرْسَلَ إِلَيَّ لاَ تَفْعَلْ.

He (the Imam) sent me a message that said, ‘You must not do so (when you are in Ruku‘).’” (Ahadith 2-9 are best explained in the form of fatwa.) صحيح - Allamah Baqir al-Majlisi (Mirʾāt al-ʿUqūl fī Sharḥ Akhbār Āl al-Rasūl (5/126))