This is part of the Responsory Benedicamus Patrem, for the feast of the Holy Trinity. As sung from late fifteenth century Antiphonary B-Gu15.
This is from the cd ‘Etienne de Liège – In festo sanctissimae Trinitatis’ RIC 249. Buy the cd at http://www.ricercar.be – other discs available as well. Check it out!