Revelation 15:3-4
And they sing the song of Moses the servant of God, and the song of the Lamb, saying, Great and marvellous are thy works, Lord God Almighty; just and true are thy ways, thou King of saints. Who shall not fear thee, O Lord, and glorify thy name? for thou only art holy: for all nations shall come and worship before thee; for thy judgments are made manifest.
Exodus 15:1-3
Then sang Moses and the children of Israel this song unto the LORD, and spake, saying, I will sing unto the LORD, for he hath triumphed gloriously: the horse and his rider hath he thrown into the sea... The LORD is my strength and song, and he is become my salvation: he is my God, and I will prepare him an habitation... The LORD is a man of war: the LORD is his name.
The victorious overcomers in Revelation 15 sing the Song of Moses — the Exodus 15 victory song after the Red Sea crossing — as the constitutional celebration of the final eschatological triumph. The Exodus 15 Song was the founding covenant victory hymn: the LORD has triumphed gloriously, thrown the enemy into the sea. Revelation 15's overcomers sing this same song as the ultimate victory anthem, establishing the eschatological triumph as the cosmic fulfillment of the Exodus 15 Red Sea victory that the original song celebrated.
Deuteronomy 32:4
He is the Rock, his work is perfect: for all his ways are judgment: a God of truth and without iniquity, just and right is he.
The Song's declaration 'just and true are thy ways, thou King of saints' also invokes the Deuteronomy 32 Song of Moses' Rock-justice declaration as its constitutional source. Moses declared the LORD's ways to be judgment, truth, and righteousness — the constitutional character description. The Revelation victory-song incorporates this Deuteronomy 32 character statement as the eschatological affirmation that all nations will ultimately acknowledge: the divine ways that Moses declared just and right are vindicated before all creation.