Albert Barnes on Luke 9:44

Let these sayings. Probably this refers to the sayings of the people, who had seen his miracles, and who on that account had praised and glorified God. On that ground they had acknowledged him to be the Christ. As if he had said, "I am about to die. You will then be disconsolate, and perhaps doubtful about my being the Christ. Then do you remember these miracles, and the confessions of the people--the evidence which I gave you that I was from God." Or it may mean, "Remember that I am about to die, and let my sayings in regard to that sink down into your hearts, for it is a most important event; and you will have need of remembering, when it takes place, that I told you of it." This last interpretation, however, does not agree as well with the Greek as the former.

(m) "for the Son of man" Mt 17:22