Prophet Muhammad (SAW) told the Companions about the Dajjal. He explained in such a way that the Companions expected the Dajjal to be there in their time. Prophet Muhammad (SAW) saw this on their faces and asked what is the matter? They told him that they thought the Dajjal was going to come now (in their time). Prophet Muhammad made the point that the Dajjal can come out in any time. Only Allah (SWT) knows when it will be. A powerful reminder.