Albert Barnes on Matthew 26:74

Then began he to curse, etc. Peter was now irritated beyond endurance. He could no longer resist the evidence that he was known. It had been repeatedly charged on him. His language had betrayed him, and there was a positive witness who had seen him. He felt it necessary, therefore, to be still more decided; and he accordingly added to the sin of denying his Lord, the deep aggravation of profane cursing and swearing; affirming, what he must have known was false, that he knew not the man. Immediately then the cock crew; that is, the second crowing, or not far from three in the morning.