Kabbalah zu Schemot 34:33

When Moses was taken before Pharaoh, God took it from him, and did not give it again until he stood upon the mount of Sinai to receive the Torah. Thenceforth Moses had it for his until the end of his life, and therefore he could not be approached by the Israelites until he had put a veil upon his face {Exod. 34:33}.