They held harps given them by God

And I saw what looked like a sea of glass mixed with fire and, standing beside the sea, those who had been victorious over the beast and his image and over the number of his name.

They held harps given them by God and sang the song of Moses the servant of God and the song of the Lamb:

“Great and marvelous are your deeds,
Lord God Almighty.
Just and true are your ways,
King of the ages.
Who will not fear you, O Lord,
and bring glory to your name?
For you alone are holy.
All nations will come
and worship before you,
for your righteous acts have been revealed.”
Revelation 15:2-4