John Chrysostom

What then do the Pharisees? "When they saw it," it is said, "they said unto Him, Behold, Thy disciples do that which is not lawful to do upon the Sabbath day." Now here indeed with no great vehemence yet surely that would have been consistent in them)-nevertheless they are not vehemently provoked, but simply find fault. But when He stretched out the withered hand and healed it, then they were so infuriated, as even to consult together about slaying and destroying Him. …

Homily on the Gospel of Matthew 39 ↗

Jerome

But the Pharisees, seeing this, said to him: Behold, your disciples do what is not lawful for them to do on the Sabbath. Note that the first apostles of the Savior destroy the letter of the Sabbath against the Ebionites, who, while accepting the other apostles, reject Paul as a transgressor of the law.

Commentary on Matthew ↗

Hilary of Poitiers

The Pharisees, who thought that the key of the kingdom of heaven was in their hands, accused the disciples of doing what was not lawful to do; whereon the Lord reminded them of deeds in which, under the guise of facts, a prophecy was concealed; and that He might show the power of all things, He further added, that it contained the form of that work which was to be, Had ye known what that meaneth, I will have mercy; for the work of our salvation is not in the sacrifice of the Law, but in mercy, and the Law having ceased, we are saved …

Catena Aurea by Aquinas ↗

Thomas Aquinas

Then he presents the rebuke against the Pharisees. But when the Pharisees saw it, they said to him: "Look, your disciples are doing what is not lawful to do on the Sabbath." The disciples were doing two things: first, they plucked someone else's grain; secondly, they were violating the Sabbath. But the Pharisees did not complain about the first, because it was permitted by the Law (Dt c. 23); therefore, because it was permitted, they were not stopped; but because it was on the Sabbath, they were judged maliciously. …

Commentary on Matthew ↗