Adam Clarke on Acts 22:5

The high priest doth bear me witness, etc. - He probably referred to the letters of authority which he had received from the high priest, and the whole estate of the elders, παν το πρεσβυτεριον, the whole of the presbytery, that is, the sanhedrin; and it is likely, that he had those letters to produce. This zeal of his against Christianity was an ample proof of his sincerity as a Pharisaical Jew.