The oldest prayer we know of dedicated to the Virgin Mary is known as "Beneath thy compassion" (Greek: Ὑπὸ τὴν σὴν εὐσπλαγχνίαν; Latin: Sub tuum praesidium). The earliest text of this hymn was found in a Coptic Christmas liturgy of the third century. It is written in Greek and dates to approximately 250 A.D. It is used in the Coptic liturgy to this day, as well as in the Orthodox, Ambrosian, and Latin liturgies.
For Orthodox, the hymn is sung as the last dismissal hymn of daily Vespers during Great Lent.
The entire hymn in Greek reads:
Ὑπὸ τὴν σὴν εὐσπλαγχνίαν,
καταφεύγομεν, Θεοτόκε.
Τὰς ἡμῶν ἱκεσίας,
μὴ παρίδῃς ἐν περιστάσει,
ἀλλ᾽ ἐκ κινδύνων λύτρωσαι ἡμᾶς,
μόνη Ἁγνή, μόνη εὐλογημένη.
In English:
Beneath thy compassion,
We take refuge, O Mother of God:
do not despise our petitions in time of trouble,
but rescue us from dangers,
only pure one, only blessed one.
Read more about this hymn here.
At the three minute mark of the video below, the hymn is chanted in the context of a Vespers Service for Great Lent.