When He opened the fifth seal, I saw under the altar the souls of those who had been slain for the word of God and for the testimony which they held.
Revelation 6:9
Surrounding context
6:8 So I looked, and behold, a pale horse. And the name of him who sat on it was Death, and Hades followed with him. And power was given to them over a fourth of the earth, to kill with sword, with hunger, with death, and by the beasts of the earth.
6:9 When He opened the fifth seal, I saw under the altar the souls of those who had been slain for the word of God and for the testimony which they held.
6:10 And they cried with a loud voice, saying, "How long, O Lord, holy and true, until You judge and avenge our blood on those who dwell on the earth?"