This is a Chant piece sung before Mass on the 1st Sunday of Advent (in some places). It tells us where Gregorian Chant came from, and how highly recommended it is :)

Sanctissimus namque Gregorius
cuм preces effunderet ad Dominum
ut musicuм bonum ei desuper
in carminibus dedisset.
Tunc descendit Spiritus Sanctus super eum
in specie columbae
et illustravit cor eius,
et...
sic demum exorsus est canere
ita dicendo:

Ad te levavi animam meam:
Deus meus in te confido, non erubescam:
neque irrideant me inimici mei:
etenim universi
qui te exspectant, non confundentur.

V. Vias tuas, Domine demonstra mihi:
R. et semitas tuas edoce me.


Translation:

Once, the most holy Gregory
was pouring out prayers to the Lord
that he might be given
good music in song from on high.
Then the Holy Ghost descended on him
in the form of a dove,
and enlightened his heart,
and...
at length, he was moved to sing
saying thus:

To Thee, have I lifted up my soul:
My God, in thee have I hoped, I will not be put to shame:
Neither will my enemies laugh at me:
For all those
who hope in Thee, shall not be confounded.

V. Thou hast shown me thy ways, O Lord:
R. and have taught me thy paths.