Albert Barnes Commentary Matthew 12:3

"But he said unto them, Have ye not read what David did, when he was hungry, and they that were with him;" — Matthew 12:3 (ASV)

But he said to them, etc. To vindicate his disciples, he referred them to a similar case, recorded in the Old Testament, and therefore one with which they ought to have been acquainted. This was the case of David. The law commanded that twelve loaves of bread should be laid on the table in the holy place in the tabernacle, to remain a week, and then to be eaten by the priests only.

Their place was then supplied by fresh bread. This was called the shew-bread (Leviticus 24:5–9). David, fleeing before Saul, weary and hungry, had come to Ahimelech the priest; had found only this bread; had asked for it, and had eaten it, contrary to the letter of the law (1 Samuel 21:1–7).

David, among the Jews, had high authority. This act had passed uncondemned. It proved that in cases of necessity the laws did not bind a person: a principle which all laws admit. So the necessity of the disciples justified them in doing on the Sabbath what would have been otherwise unlawful.