He then lying on Jesus' breast - This is a different word from the one rendered John 13:23 leaning. It means falling back or reclining on the bosom of Jesus. When Peter spake, John laid his head back on the bosom of Jesus, so that he could speak to him privately without being heard by others.

He then lying on Jesus' breast - Επιπεσων, laying his head against the breast of Christ, in a loving, respectful manner. As the expressions in the text are different here from those in the preceding verse, it shows that John altered his position at table, in order to ask the question which Peter suggested, which he probably did by whispering to our Lord; for, from John 13:28, we may learn that the other disciples had not heard what John said; and it is likely that the following words - It is he to whom I shall give the morsel when I have dipped it, were whispered back by Christ to John.

He then lying on Jesus' breast,.... Being on the couch just before Jesus, with his back to him, he bends backwards, and falling on Jesus' breast, whispers in his ear:
and saith unto him, Lord, who is it? using his interest in Christ, and making thus free with him, in compliance with Peter's request; and was no doubt desirous himself of knowing who the person was.

He then lying--rather leaning over on Jesus' bosom.
saith--in a whisper, "Lord, who is it?"

Leaning down, and so asking him privately.