Papyrus Oxyrhynchus (P. Oxy. XV 1786) is the earliest known papyrus in Greek, containing one of the earliest Christian hymns with music notation, dating from the 3rd century AD. The papyrus was found in 1918 and published in 1922.
It is notable for containing an explicit Trinitarian formula:
“As we sing praises to the Father and the Son and the Holy Spirit, may all powers cry out: Amen, amen. The power, the honor and the glory forever be to God, the only giver of all that is good. Amen, amen. ”
“Phos Hilaron” (Greek Φῶς Ἱλαρόν, literally “Gladsome Light”) is a prayer chant dating from the second half of the 2nd century AD, considered to be the most ancient Christian hymn. Notably, this hymn, sung in the evening, during the lighting of the lamps, also contains a Trinitarian formula:
“O Gladsome Light of the holy glory of the immortal, heavenly, holy, blessed Father, O Jesus Christ: Having reached the time of sunset, having seen the evening light, we sing of the Father and the Son and the Holy Spirit, God. Meet it is for Thee at all times to be hymned with reverent voices, O Son of God, Giver of life. Wherefore, the world doth glorify Thee.”
The first written mention refers to the Apostolic Constitutions (late III-early IV century). The earliest text of the hymn is in the Codex Alexandrinus (5th century), before the Psalms. 
It is noteworthy that that St Basil the Great appealed to this ancient hymn (as to a tradition known to everyone in the church) in the context of the existing controversy about the Deity of the Holy Spirit:
“Our fathers refused to receive the grace of the evening light in silence, but immediately, as it came, they brought thanksgiving. Although we cannot be sure who was the creator of these praises that we read during the Lamp-lighting prayers, the people repeat the ancient voice, and no one has yet entertained the idea that they say blasphemy when they say: we praise the Father, the Son and the Holy Spirit, God. If anyone knew the hymn that Athenogenes left to his disciples as preventive medicine when he himself hastened to the burnt offering, he would also know what the martyrs thought about the Spirit” (St Basil the Great, to Amphilochius About the Holy Spirit, chapter 29).
As we see, the ancient Christian hymns are witnesses to the actual religious practice of worship of ante-Nicene Christianity, which refutes the widespread belief about the more recent origin of worshipping the Trinity.
 Quoted from: Cosgrove, Charles H. 2011. An Ancient Christian hymn with Musical Notation: Papyrus Oxyrhynchus 1786. Mohr Siebeck. P. 34
 Vassiliadis, Petros. “From the Pauline Collection to Phos Hilaron of Cappadocia”, p. 4-5.
Translated by The Catalogue of Good Deeds