How should one place the hands in the bowing position (*Ruku'*):
Chapter on the Description of Prayer
Al-Mughni
Book of Prayer
Primary text
When bowing, the Messenger of Allah, peace be upon him, placed his palms upon his knees. In this posture, the back must be straightened and level, neither letting the head dip down nor drawing the chin in. The position requires that every vertebra returns to its place.
Supporting text
In one narration, it is specified that he 'firmly placed his hands on his knees' and 'leaned his back until it was level.'