Peter turned around and saw the disciple whom Jesus loved [i.e., probably the apostle John] following [along behind him]. This was the same disciple who had leaned over close to Jesus at the [Passover] meal and asked Him, "Lord, who is it that will turn you over [to the Jewish authorities]?"

Peter having seen this one, saith to Jesus, 'Lord, and what of this one?'

Jesus saith to him, 'If him I will to remain till I come, what -- to thee? be thou following me.' This word, therefore, went forth to the brethren that that disciple doth not die,

So this report spread to the brothers that this disciple would not die. Yet Jesus did not tell him that he would not die, but, “If I want him to remain until I come, what is that to you?”

This is the disciple who testifies to these things and who wrote them down. We know that his testimony is true.