Exodus 6:9-12
"Moses reported this to the Israelites, but they did not listen to him because of their discouragement and harsh labor. Then the LORD said to Moses, 'Go, tell Pharaoh king of Egypt to let the Israelites go out of his country.' But Moses said to the LORD, 'If the Israelites will not listen to me, why would Pharaoh listen to me, since I speak with faltering lips?'" (NIV)