Matthew 26:22 - Ellicott's Commentary On The Whole Bible

They were exceeding sorrowful. — St. John (John 13:22) describes their perplexed and questioning glances at each other, the whisper of Peter to John, the answer of our Lord to the beloved disciple, announcing the sign by which the traitor was to be indicated. All this passed apparently as a by-play, unheard or unheeded by the other disciples. It was followed by the hands of the Master and the Traitor meeting in the dish (probably that which contained the conserve of fruit above referred to); and dipping a piece of the unleavened bread in the syrup, the One gave it to the other. The signal was, of course, understood by Peter and John, but probably not by the others.