Adam Clarke on John 13:26

And when he had dipped the sop - Dr. Lightfoot observes that it was no unusual thing to dip a sop and give it to any person; and it is probable that the rest of the disciples considered it as given to Judas that he might hurry to do some work on which he wished to employ him, and not wait to finish his supper in a regular manner. They did not hear the question that John asked, nor our Lord's answer; but they no doubt heard the words, That thou doest do quickly - and might understand them as above.