מדרש על ברכות 9:1

And he led the flock until he came to Horeb, as it is said, "And he led the flock to the back of the wilderness, and came to the mountain of God, unto Horeb" (Ex. 3:1). There the Holy One, blessed be He, was revealed unto him from the midst of the thorn-bush. Moses saw the bush burning with fire, and the fire did not consume the bush, and the bush did not extinguish the flames of fire. Now the bush does not grow in the earth unless it has water beneath it. Moses saw and was wondering very much in his heart, and he said: What kind of glory is there in its midst? He said: I will now turn aside and see this great sight, why the thorn-bush is not burnt. The Holy One, blessed be He, said to him: Moses ! Stand where thou art standing, for there in the future will I give the Torah to Israel, as it is said, "And he said, Draw not nigh hither; put off thy shoes from off thy feet, for the place whereon thou standest is holy ground" (Ex. 3:5). The Holy One, blessed be He, said to him: Go. Hence (the sages) said: Anyone who enters the Temple must remove || his shoe, for thus spake the Holy One, blessed be He, to Moses: "Put off thy shoes from off thy feet" (ibid.).