John Chrysostom

But that none should say, For this cause am I slack to practise, because my instructor is evil, He removes every such plea, saying, All therefore whatsoever they say unto you, that observe and do, for they speak not their own, but God's, which things He taught through Moses in the Law. And look with how great honour He speaks of Moses, showing again what harmony there is with the Old Testament.

Catena Aurea by Aquinas ↗

Jerome

Then Jesus spoke to the crowds and to his disciples, saying: The scribes and Pharisees have seated themselves on the chair of Moses. Therefore, whatever they tell you, observe and do it, but do not do their works. For they speak, but do not practice. What is gentler, what is kinder than the Lord? He is tempted by the Pharisees, their plots are crushed, and according to the Psalmist: The arrows of little children have become their wounds (Psalm 63:8). …

Commentary on Matthew ↗

Thomas Aquinas

Then he admonishes them to obedience with caution; and he does two things. First, he exhorts them to obey; secondly, to beware. All things whatsoever they shall say to you, observe, namely in the heart, and do, in deed; Deuteronomy 17:9: you shall come to the priests of the levitical race, and to the judge; and afterward: and you shall do whatsoever they shall say; and it follows: you shall follow their sentence. And the Apostle says: obey your prelates. And this is against the Manicheans, who said that the old law was not good. …

Commentary on Matthew ↗