Exodus 3:5

Put off thy shoes - The reverence due to holy places thus rests upon God’s own command. The custom itself is well known from the observances of the temple, it was almost universally adopted by the ancients, and is retained in the East.

Holy ground - This passage is almost conclusive against the assumption that the place was previously a sanctuary. Moses knew nothing of its holiness after some 40 years spent on the Peninsula. It became holy by the presence of God.