John 18:21
“Why do you ask Me? Ask them which heard Me.”
These are not the words of one speaking arrogantly, but of one confiding in the truth of what He had said. What therefore He said at the beginning, “If I bear witness of Myself, My witness is not true”, this He now implies, desiring to render His testimony abundantly credible. For when Annas mentioned the disciples, what says He? “Do you ask Me concerning Mine? Ask Mine enemies, ask those who have plotted against Me, who have bound Me; let them speak.” This is an unquestionable proof of truth, when one calls his enemies to be witnesses to what he says. What then does the high priest? When it would have been right thus to have made the enquiry, that person did not so.
Source: Homilies on the Gospel of John (New Advent)