"Mirabilem te, Deus, et unum Sanctum in omnibus Sanctis tuis. . . ."
Post communion, Feast of All Saints, Roman missal, but before that the Missale Parisiense of 1738 and I think (though I haven't double-checked) 1706 (ed. Noailles). See Corpus orationum 13, p. 55, no. 3798. There, i.e. in the Introduction to the Missale Parisiense on p. 3, it is said that many of the prayers in the Missale Parisiense go back to ancient models, e.g. ancient prefaces and episcopal benedictions. Thus, my guess would be that this post communion was constructed on the basis of the antiphon present from 935+ in the database Cantus (see, for example, the presence of that antiphon in this manuscript Missale Parisiense of 1497), itself a quotation of Ps 67:36 iuxta LXX ("mirabilis Deus in sanctis suis").