The Introit for the third Sunday of Advent is from Philippians 4:4-6 — “Rejoice in the Lord alway: and again I say, Rejoice: let your moderation be known unto all men: the Lord is at hand. Be careful for nothing: but in every thing, by prayer and supplication, with thanksgiving, let your requests be made known unto God.”
This is followed by the first verse from Psalm 85: “Lord, thou art become gracious unto thy land: thou hast turned away the captivity of Jacob.”
Here is the Latin text:
Gaudete in Domino semper: iterum dico, gaudete. Modestia vestra nota sit omnibus hominibus: Dominus enim prope est. Nihil solliciti sitis: sed in omni oratione et obsecratione cum gratiarum actione petitiones vestræ innotescant apud Deum. Benedixisti Domine terram tuam: avertisti captivitatem Jacob.
Here is the chant of this text from the Roman Gradual, the official liturgical book of the Roman Rite of the Roman Catholic Church.