Surely you can go no further back than the New Testament. After learning the news that she is to be the mother of Jesus, Mary goes to minister to her cousin Elizabeth, who is expecting John the Baptist. Here are the words of Elizabeth's greeting to Mary from Luke 1:42-43:
Εὐλογημένη σὺ ἐν γυναιξίν, καὶ εὐλογημένος ὁ καρπὸς τῆς κοιλίας σου. καὶ πόθεν μοι τοῦτο ἵνα ἔλθῃ ἡ μήτηρ τοῦ κυρίου μου πρὸς ἐμέ;
Jeromes's Vulgate translation is as follows:
Benedicta tu inter mulieres, et benedictus fructus ventris tui. Et unde hoc mihi ut veniat mater Domini mei ad me?
I'm not sure if your question is about the notion of calling Mary "Mother of the Lord" or rather the exact three-word phrase in Latin. The latter question is a bit tough because (as far as I'm aware) this wasn't ever a common Marian formula. You would have much more luck finding references to the "Θεοτόκος" (God-bearer), "Mater Dei," or "Dei Genitrix."