Answered by Shaykh Faraz Rabbani
Question: As-Salamu alaikum,
I understand that whilst in the ruku position in prayer one’s back has to be parallel to the floor. My question is what if one is unable to do bow down to that extent? Would a lesser ruku be valid?
Answer: walaikum assalam wa rahmatullah,
I pray this finds you in the best of health and spirits.
The minimal bowing (ruku) is for one’s hands to be able to reach the knees–as this is what is minimally considered “bowing.” Allah Most High simply commands us to, “Bow and prostrate.” [Qur’an, 22.77]
The optimal bowing, for men, includes keeping one’s back straight, parallel to the ground. This is sunna, not obligatory: there is reward in bowing in this way, but the validity of one’s bowing and prayer is not dependent upon it. [Shurunbulali, Maraqi al-Falah]