Charles Ellicott Commentary Jeremiah 51:62

"and say, O Jehovah, thou hast spoken concerning this place, to cut it off, that none shall dwell therein, neither man nor beast, but that it shall be desolate for ever." — Jeremiah 51:62 (ASV)

Then you shall say, O Lord ... —The words which recite the predictions of Jeremiah 50:3, 39 are of the nature of an implied prayer, reminding Jehovah of what He had promised, and entreating Him to fulfil it. “You have said” was to be the ground of the suppliant’s prayer for the fulfilment.