John Gill Commentary 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?" — Mark 2:25 (ASV)

And he said unto them
By way of answer to their question, and which was a full one, and enough to silence them:

have you never read what David did ;
referring to the history in (1 Samuel 21:1–15) .

when he had need :
of bread, was in great necessity, and in the utmost distress:

and was an hungered, he, and they that were with him ?
which was a justifiable reason for what he and his company did; as it was for the action of the disciples; being in a like case, and therefore very appropriate to the purpose; (See Gill on Matthew 12:3).