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 the first verse of Psalm 70 (KJV: "Make haste, O God, to deliver me; Make haste to help me, O LORD."). 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.
Read more about Deus In Adjutorium Meum Intende: Liturgical Use