Adam Clarke on Matthew 23:3

All therefore whatsoever - That is, all those things which they read out of the law and prophets, and all things which they teach consistently with them. This must be our Lord's meaning: he could not have desired them to do every thing, without restriction, which the Jewish doctors taught; because himself warns his disciples against their false teaching, and testifies that they had made the word of God of none effect by their traditions. See Mat 15:6, etc. Besides, as our Lord speaks here in the past tense - whatsoever they Have commanded, ὁσα ειπωσιν, he may refer to the teaching of a former period, when they taught the way of God in truth, or were much less corrupted than they were now.