"and knelt down looking up at the Blessed Sacrament and the choir began to sing Tantum ego" (U13.497)
'Tantum ergo' are the opening words of the last 2 verses of Pange Lingua, a Mediaeval Latin hymn written by St Thomas Aquinas. The two verses are sung during the Veneration and Benediction of the Blessed Sacrament in the Roman Catholic Church.
The Tantum ergo starts:
Tantum ergo Sacramentum (Down in adoration falling)
Veneremur cernui... (The sacred Host we venerate)...