Dominus Vobis cum
(Latin: The Lord be with you)
A blessing found in Ruth 2:4, and occurring trequently in Scripture with slight variation, notably in the salutation of the Angel Gabriel to the Virgin Mary (Luke 1:28).
It occurs eight times in the Mass, four times as a salutation from priest to people, and before the principal prayer in each hour of the Office.
The answer, "Et cum spiritu tuo" (And with thy spirit), formerly an acclamation by the entire congregation at Mass, is also scriptural (2 Timothy 4:22).
New Catholic Dictionary