I believe the first to mention it is Epiphanios of Salamis (d. 403). He says: “If anyone holds that we are mistaken, let him simply follow the indications of Scripture, in which is found no mention of Mary’s death, whether she died or did not die, whether she was buried or was not buried. For when John was sent to his voyage to Asia, no one says that he had the holy virgin with him a companion. Scripture simply is silent, because of the greatness of the prodigy, in order to strike the mind of man with excessive wonder. As far as I am concerned, I dare not speak out, but I maintain a meditative silence. For you would find (in Scripture) hardly any news about this holy and blessed woman, of whom nothing is said concerning her death.... But I dare not affirm this with absolute certainty, nor do I say that she remained untouched by death, nor can I confirm whether she died. The Scriptures, which are above reason, left this question so that no one could suspect her of carnal baseness. We not know if she died or if she was buried; however, she did not have carnal relations. Let this never be said!” (PG 42:716).” He later wrote: “If the holy Virgin is dead and has been buried, surely her dormition happened with great honor; her end was most pure and crowned with virginity. If she was slain, according to what is written, ‘A sword shall pierce your soul’, then she obtained glory together with the martyrs and her holy body, from which light shone forth for all the world, dwells among those who enjoy the repose of the blessed. Or she continued to live. For, to God, it is not impossible to do whatever he will; on the other hand *no one knows exactly what her end was.*” (PG 42:737).
None of the earlier fathers mention it, at least that I have discovered. And when someone is confessing in 400 that no one knows her exact end...the later certainty regarding the matter is what this Lutheran would call HIGHLY suspect. (John of Damascus, Germanus, Andrew of Crete, they will all be quite certain and quite late). You’ll not read hide nor hair of it in Ignatius, Irenaeus, Justin, Augustine, Ambrose, Hilary, Basil, Chrysostom, etc.