Matthew 5:21

In each of these six cases Jesus first related the popular understanding of the Old Testament, the view advocated by the religious teachers of His day. In this verse He introduced it by saying, "You have heard that the ancients were told" (NASB). This was an expression that the rabbis of Jesus’ day used when they referred to the teachings of the Old Testament. [Note: D. Daube, The New Testament and Rabbinic Judaism, p. 55.]

Jesus quoted the sixth commandment and combined it with Leviticus 19:17. The "court" in view was the civil court in Israel.