This Gregorian chant, Amen dico vobis, is the appointed Communion Proper for the Twentythird Sunday After Pentecost, or, Thirtythird Sunday of the Novus Ordo year.
The sacred text is taken from the Holy Gospel according to Saint Mark 11:24
"Therefore I say unto you: what things soever ye desire, when ye pray, believe that ye receive them, and ye shall have them."