According to the Bible, Moses had a speech impediment. When God told Moses to free the Israelites (Jews) from slavery in Egypt, Moses replied, "But my Lord, never in my life have I been a man of eloquence, either before or since you have spoken to your servant. I am a slow speaker and not able to speak well" (Exodus 4:10). After reassuring Moses that he was the man for the job, God told him that his brother Aaron the Levite would speak to the Israelites on Moses's behalf.
Further Information: Goldman, Elizabeth. Believers: Spiritual Leaders of the World.New York: Oxford University Press, 1995; "Moses." Catholic Encyclopedia. [Online] Available http://www.newadvent.org/cathen/10596a.htm, October 20, 2000.