Revelation 6:9-11

9When the Lamb opened the fifth seal, I saw under the altar the souls of those slaughtered for the sake of the word of God and for the witness they had.

10And they cried out with a loud voice, saying, “O Sovereign Master, holy and true, how long before You judge those who dwell on the earth and avenge our blood?”

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