G‑d tells Moses and Aaron to take a staff and gather the people in front of a certain rock. They should speak to the rock, and it will give forth water. Moses and Aaron gather everybody, and Moses strikes the rock and it gives forth water. In the course of this episode they committed a grave error, the conventional explanation being that they struck the rock instead of speaking to it. This caused G‑d to punish Moses and Aaron, barring them from leading the Jews into Israel.