St. Thomas Aquinas's Vespers hymn for the Office of Corpus Christi — 'Sing, my tongue, the Saviour's glory' — whose last two stanzas form the Tantum Ergo sung at Benediction.
Sing, my tongue, the Saviour's glory, of his Flesh the mystery sing; of the Blood, all price exceeding, shed by our immortal King, destined, for the world's redemption, from a noble womb to spring. […]
Down in adoration falling, lo! the sacred Host we hail; lo! o'er ancient forms departing, newer rites of grace prevail; faith, for all defects supplying, where the feeble senses fail. — These last two stanzas, the Tantum ergo Sacramentum, are sung at Benediction of the Blessed Sacrament.
The dated sources the library holds, oldest first — in their own words.
First kept at Liège in 1246, the feast of Corpus Christi is extended to the whole Church by Pope Urban IV in 1264 (the bull Transiturus); St. Thomas Aquinas composes its Office — the hymns for the hours and the sequence for the Mass.
“The former, in unrhymed verse, is generally credited to St. Venantius Fortunatus (6 cent.), and the latter, in rhymed accentual rhythm, was composed by St. Thomas Aquinas (13 cent.).”
The two closing stanzas of the Pange Lingua — "Tantum ergo Sacramentum" — became the hymn sung at Benediction of the Blessed Sacrament, among the most familiar of all Eucharistic chants.