Exodus 4:6
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 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.
The verse belongs to the call and commission of Moses at the burning bush near Horeb, the mountain of God, in Midian. There God revealed Himself as the God of Abraham, the God of Isaac, and the God of Jacob, and commissioned Moses to bring Israel out of Egypt. The time for Israel's deliverance had come, and God purposed to accomplish it in a manner to pour contempt on human pride. The deliverer went forth as a humble shepherd with only a rod in his hand, and God made that rod the symbol of His power.
Moses feared the people would not believe him or listen to his voice, and pleaded for a sign that his message was from heaven. God answered by giving the leprous hand as an assuring sign that He could confirm a heavenly message. When Moses put his hand into his bosom and withdrew it, it was leprous as snow, and when he put it back and withdrew it again, it was restored like the other. It was given together with the rod becoming a serpent.
The sign assured Moses that God's power was with him. It was to convince Israel and Pharaoh that One mightier than Egypt's king was present among them. It was meant to cause Pharaoh and the Egyptians to fear so they would not dare to harm Moses, and to convince the king and his people that a power greater than Moses was manifesting itself before them. Even after many such miracles were performed before Pharaoh in the sight of the people, Pharaoh would still not let Israel go. Moses's plea for heavenly confirmation by the leprous hand is the same kind of request later made by Hezekiah.