Then Peter, turning around, saw the disciple whom Jesus loved following, who also had leaned on His breast at the supper, and said, "Lord, who is the one who betrays You?"
John 21:20
Surrounding context
21:19 This He spoke, signifying by what death he would glorify God. And when He had spoken this, He said to him, "Follow Me."
21:20 Then Peter, turning around, saw the disciple whom Jesus loved following, who also had leaned on His breast at the supper, and said, "Lord, who is the one who betrays You?"
21:21 Peter, seeing him, said to Jesus, "But Lord, what about this man?"