Lyrics:
O magnum mysterium, et admirabile sacramentum, ut animalia viderent Dominum natum, jacentem in praesepio! Beata Virgo, cujus viscera meruerunt portare Dominum Christum. Alleluia
O great mystery, and wonderful sacrament, that animals should see the new-born Lord, lying in a manger! Blessed is the Virgin whose womb was worthy to bear Christ the Lord. Alleluia!
Listen to it all.
There is a nice article on this there.