The Least Bending That Suffices for the Bowing
What is the least degree of bending by which the bowing is realised for one able to stand?
Hanafi
The least that suffices is to bend such that, were he to stretch out his hands, they would reach his knees, or that he be nearer to bowing than to standing.
Reference: Al-Hidāyah — Radd al-Muḥtār — Tabyīn al-Ḥaqāʾiq
Maliki
The least that suffices is to bend until his palms come near his knees, were he to place them; levelling the back belongs to perfection and is not a condition.
Reference: Al-Mudawwanah — Al-Sharḥ al-Kabīr — Mawāhib al-Jalīl
Shāfiʿī
For one of ordinary build, the least that suffices is to bend such that his palms would reach his knees were he to intend placing them upon them, without bending his knees.
Reference: Al-Umm — Minhāj al-Ṭālibīn — Al-Majmūʿ
Hanbali
For one standing, the least that suffices is to bend until his hands would touch his knees were he to place them upon them; levelling the back is a sunnah, not a condition.
Reference: Al-Mughnī — Al-Kāfī — Kashshāf al-Qināʿ