From Dom Guéranger's The Liturgical Year.
Let us honour the holy memory of the Bishop of Arezzo, whom the persecution of Julian the Apostate sent on this day to heaven. The following prayer, wherein the Church expresses her unchanging confidence in his powerful intercession, is found so far back as in the Gelasian Sacramentary; though the title of Confessor is there used instead of Martyr, it is beyond all question that Donatus died for Christ.
Prayer
Deus, tuorum gloria sacerdotum: præsta quæsumus; ut sancti martyris tui et episcopi Donati, cujus festa gerimus, sentiamus auxilium. Per Dominum.
O God, the glory of Thy priests, grant, we beseech Thee, that we may experience the succour of Thy holy martyr and bishop, Donatus, whose festival we celebrate. Through our Lord, etc.