Gregory the Dialogist

For Thou hast said, My speech is pure, and I am clean in Thine eyes. Whoso remembers the words of blessed Job, knows how falsely this charge is fastened upon his voice. For how could he call himself pure, who says, If I justify myself, mine own mouth shall condemn me; but there is this in the wickedness of the unrighteous, that, while it refuses to bewail real evil things in itself it invents them in others, for it makes use of it as a solace of evil doing, if the life of the reprover can be also stained with false accusations. …

Morals on the Book of Job, Book X ↗

Thomas Aquinas

But perhaps Job could say that they have no reason to reproach him or his words. To reject this he continues, "For you have said: My speech is pure." He makes this interpretation according to what Job had said already, "You will find no evil on my tongue, nor will stupidity resound on my lips" (6:30); and "I am clean in your sight." Job had not expressly said this, but Sophar wanted to take this interpretation from his words to say that Job had argued that he was not punished for sin. …

Commentary on Job ↗