John 18:25-27, "As Peter stood warming himself, he was asked, 'You are not one of His disciples, are you?' Peter denied it saying, 'I am not.' One of the priest's servants, a relative of the man whose ear Peter cut off (and Jesus healed), challenged him. 'Didn't I see you with him in the olive grove?' Again Peter denied it and at that moment a rooster began to crow."
Matthew 26:74-75 further entails, "He (Peter) began to call curses on himself and swore to them, 'I don't know the Man!' Immediately a rooster crowed. Then Peter remembered the Words Jesus had spoken, 'Before the rooster crows, you will disown me three times.' "
Mark 14:71-72, Luke 22:59-62 records the same incident.
"It is a serious sin to disown Christ, but Jesus forgave Peter (John 21:15-17). No sin is too great for Jesus to forgive if you are truly repentant. He will forgive even your worst sin if you turn from it and ask His pardon." (Bible commentary.)