20 Having turned, Peter sees the disciple whom Jesus loved following, the one who also had reclined on His bosom at the supper and said, “Lord, who is it who is betraying You?”
John 21:20 Cross References - BLB
John 13:23-26
23 There was reclining one of His disciples whom Jesus loved in the bosom of Jesus.
24 Therefore Simon Peter motions to him, to ask who it is about whom He is speaking?
25 Then he thus having leaned on the breast of Jesus, says to Him, “Lord, who is it?”
26 Then Jesus answers, “It is he to whom I will dip the morsel and will give him.” Then having dipped the morsel, He takes it and gives it to Judas, son of Simon Iscariot.
John 20:2
2 So she runs and comes to Simon Peter and to the other disciple, whom Jesus loved, and she says to them, “They have taken away the Lord out of the tomb, and we do not know where they have laid Him.”
John 21:7
7 Therefore that disciple whom Jesus loved says to Peter, “It is the Lord.” So Simon Peter, having heard that it is the Lord, put on the outer garment, (for he was naked), and he cast himself into the sea.
John 21:24
24 This is the disciple bearing witness concerning these things, and the one having written these things. And we know that his testimony is true.