Of Matthew's Declaration That, on Leaving These Parts, He Came into the Coasts of Magedan; And of the Question as to His Agreement with Mark in that Intimation, as Well as in the Notice of the Saying About Jonah, Which Was Returned Again as an Answer to Those Who Sought a Sign
106 Matthew continues as follows: “And He sent away the multitude, and took ship, and came into the coasts of Magedan;” and so on, down to the words, “A wicked and adulterous generation seeks after a sign; and there shall no sign be given unto it but the sign of the prophet Jonas.” This has already been recorded in another connection by the same Matthew. Hence again and again we must hold by the position that the Lord spoke the same words on repeated occasions; so that when any completely irreconcilable difference appears between statements of His utterances, we are to understand the words to have been spoken twice over.
In this case, indeed, Mark also keeps the same order; and after his account of the miracle of the seven loaves, subjoins the same intimation as is given us in Matthew, only with this difference, that Matthew's expression for the locality is not Dalmanutha, as is read in certain codices, but Magedan. There is no reason, however, for questioning the fact that it is the same place that is intended under both names. For most codices, even of Mark's Gospel, give no other reading than that of Magedan. Neither should any difficulty be felt in the fact that Mark does not say, as Matthew does, that in the answer which the Lord returned to those who sought after a sign, He referred to Jonah, but mentions simply that He replied in these terms: “There shall no sign be given unto it.”
For we are given to understand what kind of sign they asked— namely, one from heaven. And he has simply omitted to specify the words which Matthew has introduced regarding Jonas.
Source: The Harmony of the Gospels (New Advent)