מדרש על במדבר 21:6

"And God sent against the nation the burning snakes" (Bamidbar 21:1). Why did he punish [lit. collect from] them with snakes? Since the snake was the first to speak slander and was cursed for it, and they did not learn from it, the Holy One Blessed Be He said: The snake should come, which was the first to speak slander, and punish the speakers of slander, as it says (Ecclesiastes 10:8): "And one who breaks a fence, a snake will bit him." Another explanation: Why were they punished by snakes? The snake, even if it partakes of all the delectable foods of the world, they turn into dust in its mouth, as it says (Isaiah 65:25): "And a snake, dust is its bread" - but these [the Jews] eat man, which turns into many flavors, as it says (Psalms 106:15): "And He gave to them what they asked", and it says (Devarim 20:7): "For these forty years Hashem your God has been with you; you have not lacked anything" - the snake, which eats many things and taste one taste, should come punish these who eat one thing and taste many tastes.

(Numb. 21:6:) FIERY (serafim). <They are called seraphim> because they burn (sorefim) the soul.

Rabbi said: They slandered God again and said, We were dwelling in the land of Egypt in ease and contentment, but the Holy One, blessed be He, and Moses have brought us forth from Egypt to die in the wilderness, as it is said, "And the people spake against God, and against Moses, Wherefore have ye brought us up out of Egypt to die in the wilderness?" (Num. 21:5). What did the Holy One, blessed be He, do unto them? He sent against them fiery serpents which bit and killed them, as it is said, "And the Lord sent among the people fiery serpents, and they bit the people; and much people of Israel died" (Num. 21:6). Moses beheld the misfortune of Israel, and he arose and prayed on their behalf. The Holy One, blessed be He, said to him: Moses! Make thee a serpent of copper like that serpent || which spoke slander betwixt Adam and his helpmate, and place it on a high place. Let every man who has been bitten direct his heart to his Father who is in heaven, and let him gaze at that serpent, and he will be healed. Moses made a serpent of copper and set it up in a high place, and every man who had been bitten turned his heart to his Father who is in heaven, and gazed at that serpent, forthwith he became restored to health, as it is said, "And it came to pass, that if a serpent had bitten any man, when he looked at the serpent of copper, he lived" (Num. 21:9); and it also says, "If the serpent bite without enchantment, then is there no advantage in the master of the tongue" (Eccles. 10:11).