John 18:17
“Are you not also one of this man's disciples? And he says, I am not.”
What do you say, Peter? Did you not declare but now, “If need be that I lay down my life for You, I will lay it down”? What has happened then, that you can not even endure the questioning of a door-keeper? Is it a soldier who questions you? Is it one of those who seized Him? No, it is a mean and abject door-keeper, nor is the questioning of a rough kind. She says not, “Are you a disciple of that cheat and corrupter,” but, “of that man,” which was the expression rather of one pitying and relenting. But Peter could not bear any of these words. The, “Are you not also,” is said on this account, that John was within. So mildly did the woman speak. But he perceived none of this, nor took it into his mind, neither the first time, nor the second, nor the third, but when the cock crew; nor did this even bring him to his senses, till Jesus gave him the bitter look. And he stood warming himself with the servants of the high priest, but Christ was kept bound within. This we say not as accusing Peter, but showing the truth of what had been said by Christ.
Source: Homilies on the Gospel of John (New Advent)