출애굽기 4:4의 미드라쉬

(Moses) spake before Him: Sovereign of all worlds ! Give me a wonder or a sign. He said to him: Cast thy staff to the ground. He cast his staff to the ground, and it became a fiery serpent. Why did the Holy One, blessed be He, show unto Moses (a sign) with a fiery serpent, and why did He not show it to him with something else? But just as the serpent bites and kills the sons of man, likewise Pharaoh and his people bit and slew the Israelites. Afterwards it became again like a dry stick. Thus He spake: Likewise Pharaoh and his people shall become like this dry stick, as it is said, "And the Lord said unto Moses: Put forth thine hand, and take it by the tail" (Ex. 4:4). He spake before Him: Sovereign of all worlds ! Give me a wonder. He said to him: "Put now thine hand into thy bosom" (Ex. 4:6). And he put his hand into his bosom, and he brought it out leprous like snow. Why did the Holy One, blessed be He, show unto Moses (a sign) by means of an unclean thing, and (why) did He not show it by means of a clean thing? But just as the leper is unclean and causes uncleanliness, likewise Pharaoh and his people were unclean, and they caused Israel to be unclean. Afterwards (Moses) became clean again, and He spake to him: Likewise shall Israel become clean from the uncleanliness of the Egyptians, as it is said, "And he said, Put now thine hand into thy bosom" (ibid.).