6 But the evangelist, after saying that Annas sent Him bound unto Caiaphas, returns to the place of his narrative, where he had left Peter, in order to explain what had taken place in Annas' house in regard to his threefold denial. “But Peter was standing,” he says, “and warming himself.” He thus repeats what he had already stated before; and then adds what follows. “They said therefore unto him, Are you also one of his disciples? He denied, and said, I am not.” He had already denied once; this is the second time.
And then, that the third denial might also be fulfilled, “one of the servants of the high priest, being his kinsman whose ear Peter cut off, says, Did I not see you in the garden with him? Peter then denied again, and immediately the cock crew.” Behold, the prediction of the Physician is fulfilled, the presumption of the sick man is brought to the light. For there is no performance of what the latter had asserted, “I will lay down my life for Your sake;” but a performance of what the former had predicted, “You shall thrice deny me.” But with the completion of Peter's threefold denial, let the present discourse be also now completed, that hereafter we may make a fresh start with the consideration of what was done respecting the Lord before Pontius Pilate the governor.
Source: Tractates on the Gospel of John (New Advent)