John Gill Commentary Job 15:6

"Thine own mouth condemneth thee, and not I; Yea, thine own lips testify against thee." — Job 15:6 (ASV)

Thine own mouth condemneth thee, and not I
Or shows thee to be a wicked person, guilty of things charged upon thee; out of your own mouth you are convicted, there needs no other evidence to be brought against thee, that is sufficient: and you save me, and any other, the trouble of passing the sentence of condemnation upon thee; you have done it yourself, your own mouth is judge and jury, and brings in the verdict, and pronounces it, as well as is the witness, as follows, and is instead of a thousand witnesses, (Job 9:20) ;

yea, your own lips testify against thee ;
and therefore there were no need of producing any other testimony; what he had said showed that his talk was vain and unprofitable, unbecoming a wise man, and tending to make null and void the fear of God among men, to discourage all religious exercises, and particularly prayer before God.