Mark 2:25

And he said unto them, Did ye never read what David did, when he had need, and was hungry, he, and they that were with him?

David did indeed do an unlawful thing, to which Christ referred; but why did Jesus cite such an example? Is this to be understood to mean: "Look, David sinned; therefore it is all right for us to sin"? No! Christ was not here seeking to justify his disciples on the basis that "everybody else" is doing it. The true purpose of David's conduct being brought in here was to show how unreasonable and partial the Pharisees were in their judgments. David's flagrant violation they approved; yet they would make what Jesus' disciples did sabbath-breaking!