John Gill Commentary John 13:25

"He leaning back, as he was, on Jesus` breast saith unto him, Lord, who is it?" — John 13:25 (ASV)

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 says to 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.