Matthew 12:3 & 4
"He answered, 'Haven't you read what David did when he and his companions were hungry? He entered the house of God, and he and his companions ate the consecrated bread - which was not lawful for them to do, but only for the priests.'" NIV translation
Jesus answered the charge with a little history lesson for the Pharisees and the events that Jesus spoke of are recorded in 1 Samuel 21:1-6. On each Sabbath, twelve loaves of fresh bread were set on a table in the Holy Place. At that time, the old bread was removed and was given to the priests to eat. In Leviticus 22:10 & 11, we see that, even though the bread was given to the priests to eat, it was lawful for other Israelites to be given it to eat. David must have entered the tabernacle on the Sabbath to get the bread and so it was also accepted that he had not broken the Law in getting food for his men.
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It was widely accepted in religious tradition that David had not broken the Law concerning the consecrated bread because of the fact that him and his men had been kept from relations with women and they were on a mission for King Saul. To their way of thinking, this equated David and his men with being priests and so they accepted his actions. Why did Jesus bring up David and his actions? In effect, Jesus was pointing them back to the Scriptures and away from their traditions.