http://www.pbs.org/food/the-history-kitchen/history-sandwich/ said:
Farm laborers in rural France had been eating meat between sliced bread long before it had a name, though the sandwich likely started even earlier than that. The earliest recognizable form of a sandwich may be the Korech or “Hillel sandwich” that is eaten during Jewish Passover. Hillel the Elder, a Jewish leader and rabbi who lived in Jerusalem during the time of King Herod (circa 110 BC), first suggested eating bitter herbs inside unleavened matzo bread. The herbs symbolized the bitterness of slavery, and the bread resembled the flatbreads made in haste by the ancient Israelites as they fled Egypt. Hillel’s simple recommendation of sandwiching the two foods together may indicate that this was already a popular way of serving food in the Middle East.