Мидраш к Шмот 4:7

And God said [further to him], put your hand into your breast (Shemot 4:6). They said to him, just as when the snake badmouthed I struck it with tzara'at, as it says "you shall be more cursed than all the beasts" (Bereishit 3:14), as it is said "a blemish of tzara'at" (Vayikra 13:51) [therefore, when you badmouth, I will strike you similarly]. Rabbi Elazar said, these coins tat are in it are tzara'at, and so you too are worthy of being struck with tzara'at. And why did he put it into his breast? Because it's the way of evil speech to be said in private. And so it says, "he who slanders his friend in secret, I will destroy" (Tehillim 101:5). There is no "I will destroy" [אַצְמִית atzmit]: rather, it is tzara'at [צָרַעַת], as it is said "[the land may not be sold] permanently" [לִצְמִתֻת litzmitut], and we translate it "permanently" [לַחֲלוּטִין lachlutin]. And we teach "there is no difference between a quarantined/doubtful metzora and a confirmed metzora" (Mishna Megilla 1:7). And he put his hand into his breast and brought it out, and behold his hand was afflicted with tzara'at as snow (Shemot 4:6) - he got his, since he badmouthed. Rabbi Yehoshua Dischinan, in the name of Rabbi Levi, said: from here you may learn that everyone who unjustly suspects their fellow of something is struck in their body. And They said, return your hand to your breast (ibid.) - for what sign would this be to Yisrael? Go and tell them, just as a metzora causes impurity, so too the Mitzriyim are making you impure. And just as it is purified, so too will the Holy Blessed One purify Yisrael, as it is written "And behold his hand was afflicted with tzara'at as snow (Shemot 4:6), and of healing it is written "And he brought it out from his breast and behold it had returned [to be] as his flesh" (Shemot 4:7). Our sages said in order not to provoke insult on the flesh of Moshe, thus the hand wasnot struck with tzara'at until he had brought it out from his flesh, but for healing, from within his breast it was healed. An alternative take: from here we learn that punishments wait for the righteous to come, but the attribute of good is swift to come. "And it will be, if they do not believe these two signs" (Shemot 4:9) - why did the Holy Blessed One give him three signs? Corresponding to Avraham, Yitzchak, and Ya'akov. "And take from the waters of the Y'or" (ibid.) - alludes to the fact that by means of something that was spoke to Yisra'el, the water will be in the future turned to blood, and he will be struck by their hands, as it is written "Listen, you rebels" (Bemidbar 20:10). And he struck the rock and it brought forth, as it says "Then he struck the rock and it oozed [וַיָּזוּבוּ vayazuvu] water" (Tehillim 78:20) - "oozing" always indicates blood, as it is said "And a woman who oozes an oozing [יָזוּב זוֹב, yazuv zov] of her blood" (Vayikra 15:25). And for this reason he struck the rock twice - initially it brought forth blood, and only eventually water. With the first two signs, you find that they returned to their original state, but with the blood it never returned to how it was, since he didn't want to forgive Moshe for the sin of the water. And what sign was this for Yisrael? He said to them, with this sign will the Mitzriyim be struck originally.