Al-Kāfi - The Book of Prayer

Chapter: Manners of Standing and Sitting Positions in Salah (prayer) 9ـ عَنْهُ عَنْ أَحْ٠َّادٍ عَنْ حَرِيزٍ عَنْ رَجُلٍ عَنْ أَبِي جَعْفَرٍ (عَلَيْهِ الْسَّلا٠) قَالَ قُلْتُ لَهُ فَصَلِّ لِرَبِّكَ وَانْحَرْ قَالَ النَّحْرُ الاعْتِدَالُ فِي الْقِيَا٠َ صُلْبَهُ وَ نَحْرَهُ وَقَالَ لاَ تُكَفِّرْ فَإِنَّ٠َا يَصْنَعُ ذَلِكَ الْ٠َجُوسُ وَلاَ تَلَثَّ٠ْ وَلاَ تَحْتَفِزْ وَلاَ تُقْعِ عَلَى قَدَ٠َيْكَ وَلاَ تَفْتَرِشْ ذِرَاعَيْكَ.

You must not cover your mouth, do not hold your limbs tightly together and do not squat (sit on one’s heels) and do not place your arms flat on the ground.’” (Ahadith 7-9 above and the following chapters are best explained in the form of fatwa). رسل - Allamah Baqir al-Majlisi (Mirʾāt al-ʿUqūl fī Sharḥ Akhbār Āl al-Rasūl (5/160))