Deus in adjutorium meum intende, with the response: Domine ad adjuvandum me festina (respectively, "O God, come to my assistance" and "O Lord, make haste to help me") are versions of the first verse of Psalm 69 (KJV: "Save me, O God"). In this form they are a traditional Latin Christian prayer.
These words form the introductory prayer to every Hour of the Roman, monastic, and Ambrosian Breviaries, except during the last three days of Holy Week, and in the Office of the Dead. While they are said, or sung, all present sign themselves with the sign of the cross.
Tradition says that Benedict of Nursia introduced this custom into the monastic Office and that Gregory the Great extended it to all the Roman churches; Cassian (Coll., X, 10), however, says that from the earliest Christian times the monks used this introduction very often, probably outside of the liturgical prayers.
In placing this supplication at the beginning of every Hour the Catholic Church implores the assistance of God against distractions in prayer. In the Roman Rite the "Deus in adjutorium" is preceded in Matins by the "Domine labia mea aperies", whilst in the monastic Breviary the order is reversed. In Compline it is always preceded by the "Converte nos Deus".
In the Mozarabic liturgy the Hours commence with the triple Kyrie Eleison. In all the Latin countries north, east, and west of the Alps the introduction to the solemn Vespers of Easter Sunday was formed by the nine Kyrie Eleison and Christe Eleison of the Easter Mass. In the churches which observe the Greek Rite, the Trisagion and other prayers open the Hours.
The "Deus in adjutorium" is repeated three times during the conclusional prayers of Prime. In the monasteries Prime was finished immediately after the prayer: "Domine Deus omnipotens"; then the monks went from the choir to the chapter-room, where the Martyrology was read, and the day's work was given out; before dispersing to their several occupations they sang three times the "Deus in adjutorium", to emphasize the union of prayer and labour.