Matthew 26:72
And again he denied it with an oath: I do not know the man. The second denial with an oath: the escalation from the evasive I do not know what you mean to the direct and oath-bound I do not know the man. The oath — the swearing by God that makes the false claim even more serious — is the escalation that Matthew has been preparing for with the Sermon on the Mount's teaching against oaths (Matthew 5:33–37).