Exodus - Chapter 17

Once again the people are complaining - this time because there is no water.  God tells Moses to strike the rock and water would come out of it for the people to drink.  

The rock represents Jesus - as Paul said in 1 Corinthians 10 - they drank from the spiritual rock that accompanied them, and that rock was Christ.  And for us to be able to drink of the living water that Jesus provided it was necessary for Jesus to be struck down - just as Moses needed to strike the rock.