Maʿānī al-ʾAkhbār > The meaning of haqila, muzabana, ‘araya, mukhabarat, mukhadhara, munabadha, mulamasa, the selling of pebble and other things that were declared forbidden
Hadith #14

ونهى رسول الله صلى الله عليه وآله أن يدبح الرجل في الصلاة كما يدبح الحمار، ومعناه أن يطأطئ الرجل رأسه في الركوع حتى أخفض من ظهره. وكان صلى الله عليه وآله إذا ركع لم يصوب رأسه ولم يقنعه. معناه أنه لم يرفعه حتى يكون أعلى من جسده ولكن بين ذلك، و " الاقناع " رفع الرأس وإشخاصه، قال الله تعالى: " مهطعين مقنعي رؤسهم " والذي يستحب من هذا أن يستهوي ظهر الرجل ورأسه في الركوع لان رسول الله صلى الله عليه وآله كان إذا ركع لو صب على ظهره ماء لاستقر. وقال الصادق عليه السلام: لا صلاة لمن لم يقم صلبه في ركوعه وسجوده.

He, Allah’s blessings be upon him and his Family, also forbade that a man performs tadbeeh in prayer like how a donkey performs tadbeeh. Its meaning is that a man lowers his head during kneeling until it becomes lower than (the level of) his back. When he, Allah’s blessings be upon him and his Family, used to kneel, he did not make upright his head nor raise, but it was between that. Raising is to ascend the head and make it in an upright position. Allah, Exalted is He, said: «Racing ahead, their heads raised up» (14:43). What is recommended of this is that a man’s back and hand are leveled during kneeling, because the Messenger of Allah, Allah’s blessings be upon him and his Family, if water was poured on his back while he was kneeling, he would have remained firm. Al-Sadiq, peace be upon him, said: «There is no prayer (that is accepted) for the one who does not make firm his spine during his kneeling and prostration.»