Revelation 6:9-11

9When 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 witness they had borne.

10They cried out with a loud voice, "O Sovereign Lord, holy and true, how long before you will judge and avenge our blood on those who dwell on the earth?"

11Then they were each given a white robe and told to rest a little longer, until the number of their fellow servants and their brothers should be complete, who were to be killed as they themselves had been.