# throw yourself down
That is, "jump off this building to the ground."
# it is written
That is, "a prophet of God wrote long ago."
# so your foot does not hit a stone
This means, "so that you will not be hurt at all; not even your foot will be injured on a stone." It could also be translated as, "so that even your foot will not strike against a stone; you will not be injured at all."