Exodus 4:6-7:

And the LORD said furthermore unto him, Put now thine hand into thy bosom. And he put his hand into his bosom: and when he took it out, behold, his hand was leprous as snow.
And he said, Put thine hand into thy bosom again. And he put his hand into his bosom again; and plucked it out of his bosom, and behold, it was turned again as his other flesh.

Notes on Exodus 4:6-7:

This “sickening” and “healing” was a sign and a wonder for people to see in case they questioned Moses’s authority.  They would be able to see something impossible happen and know that God had sent Moses.  Please note that God did not leave Moses sick!  The leprosy was a very temporary sign and God healed it instantly.  To leave Moses sick would be a sign that would misrepresent God’s character.

See also:

Condition: Leprosy