Exodus 4:6

מצרעת כשלג LEPROUS AS SNOW — It is the nature of leprosy to be white, and thus we read in reference to a leper, (Leviticus 13:4) “If the bright spot be white [in the skin of his flesh]”. By this sign, too, he indicated that he had made a slanderous statement when he said, (v. 1) “But, behold, they will not believe me”. Therefore He smote him with leprosy (Exodus Rabbah 3:13) even as Miriam was so smitten for the slander she had spoken (cf. Shabbat 97a).

וַיֹּ֩אמֶר֩ יְהֹוָ֨ה ל֜וֹ ע֗וֹד הָֽבֵא־נָ֤א יָֽדְךָ֙ בְּחֵיקֶ֔ךָ וַיָּבֵ֥א יָד֖וֹ בְּחֵיק֑וֹ וַיּ֣וֹצִאָ֔הּ וְהִנֵּ֥ה יָד֖וֹ מְצֹרַ֥עַת כַּשָּֽׁלֶג׃

And the Lord said furthermore to him, Put now thy hand into thy bosom. And he put his hand into his bosom: and when he took it out, behold, his hand was diseased, as white as snow.