Al-Kāfi
Hadith 12129ـ أَحْمَدُ بْنُ إِدْرِيسَ عَنْ مُحَمَّدِ بْنِ أَحْمَدَ عَنْ يَعْقُوبَ بْنِ يَزِيدَ عَنِ ابْنِ أَبِي عُمَيْرٍ عَنْ عَلِيِّ بْنِ عُقْبَةَ قَالَ رَآنِي أَبُو الْحَسَنِ (عَلَيْهِ الْسَّلام) بِالْمَدِينَةِ وَأَنَا أُصَلِّي وَأَنْكُسُ بِرَأْسِي وَأَتَمَدَّدُ فِي رُكُوعِي فَأَرْسَلَ إِلَيَّ لاَ تَفْعَلْ.
9. Ahmad ibn Idris has narrated from Muhammad ibn Ahmad from Ya’qub ibn Yazid from Ibn Abi ‘Umayr from Ali ibn ‘Uqbah who has said the following: “Once abu al-Hassan (a.s.), saw me, in al-Madinah when I performed Salah (prayer). I bent down my head and stretched in my Ruku‘ (bowing down on one’s knees). 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.)