וַיֹּ֗אמֶר הָשֵׁ֤ב יָֽדְךָ֙ אֶל־חֵיקֶ֔ךָ וַיָּ֥שֶׁב יָד֖וֹ אֶל־חֵיק֑וֹ וַיּֽוֹצִאָהּ֙ מֵֽחֵיק֔וֹ וְהִנֵּה־שָׁ֖בָה כִּבְשָׂרֽוֹ׃
And He said: ‘Put thy hand back into thy bosom.—And he put his hand back into his bosom; and when he took it out of his bosom, behold, it was turned again as his other flesh.—
Read kabbalistic (mystical) interpretation of Exodus 4:7: and classical mystical sources.