80s TV episode where a kid hears the story of David & Goliath then has a bad dream about accidentally killing a bully

I saw something on TV in the late 80s where a kid is talking to someone (his older sister I think) about a big kid who's bothering him, and then she tells him the story about David & Goliath.. Later  he sees this guy and slingshots a rock which hits him in the forehead. He exclaims something like "Bam! right between the eyes" and then realizes he killed him, and becomes very remorseful.. then he wakes up to find that he was just having a bad dream.

Anyone know what show this was?