John Fenton has an incredible post on the oldest Petitionary Prayer to Mary. From that post, a cite from Nicholas Ayo, CSC The Haily Mary: A Verbal Icon of Mary (University of Notre Dame Press, 1994):
Petitionary prayer to Mary characteristic of the second part of the Ave Maria can be found in a fragmentary way very early in the liturgical practice of the church of Alexandria. In a subterranean sanctuary dating from third-century Alexandria there is a fresco depicting the marriage at Cana with an inscription to “Holy Mary” (Haghia Maria). The Sub Tuum Praesidium is the oldest Marian prayer, cherished in the liturgy both of the East and the West. It is remarkable because of its appeal to the intercession of Mary. The Greek text was discovered in the twentieth century on a fragment of papyrus estimated to date from the third century.*
Sun [sic?] tuum praesidium confugimus,
Sancta Dei Genetrix; (Theotokos)
Nostras deprecationes ne despicias in necessitatibus,
Sed a periculis cunctis libera nos semper,
Virgo gloriosa et benedicta.
We seek refuge under your protection,
Holy mother of God;
Do not turn away our prayers in our need,
But always deliver us from all danger,
O glorious and blessed Virgin.
*The original text in Greek is a fragmentary piece of papyrus, and some reconstruction was required to present a coherent text for publication. Various liturgies, both East and West, have further adapted the text of this prayer to their particular devotional situation. There is thus no one standard Greek text to which everyone subscribes. For an exhaustive treatment of the “Sub Tuum,” see Giamberardini, Il Culto Mariano in Egitto, I:69-97 and 273.
To which he adds his own following observations:
1. The dating of this particular Marian petitionary prayer is third-century; i.e., during the time of persecution and prior to Constantine’s edict.
2. The place of the particular fragment is Alexandria. So the prayer is presumably known (and prayed?) by St Athanasius, St Clement and St Cyril. Elsewhere in his work Ayo provides evidence that the latter spoke along similar themes as found in the prayer.
3. The prayer is not confined to the East, but also found in the West. Furthermore, it is not confined to “private devotions” but is located in the liturgy (presumably the Divine Office).
Holy Mother of God, pray for us.