Answered by Shaykh Faraz Rabbani
What is the proof that one should look at between ones feet during ruku?
Walaikum assalam wa rahmatullah,
It is simply a practical legal application of the sunna of lowering one’s gaze in prayer, and the sunna of keeping one’s head level with the end of one’s back, both of which are rigorously established from the Messenger of Allah (peace and blessings be upon him).
And Allah alone gives success.