Albert Barnes Commentary Exodus 4:6

"And Jehovah said furthermore unto 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 leprous, as [white as] snow." — Exodus 4:6 (ASV)

Leprous: The instantaneous production and cure of the most malignant and subtle disease known to the Israelites was a sign of their danger if they resisted the command, and of their deliverance if they obeyed it. The infliction and cure were always regarded as special proofs of a divine intervention.