Revelation 6:9-11

9Now when the Lamb opened the fifth seal, I saw under the altar the souls of those who had been violently killed because of the word of God and because of the testimony they had given.

10They cried out with a loud voice, “How long, Sovereign Master, holy and true, before you judge those who live on the earth and avenge our blood?”

11Each of them was given a long white robe and they were told to rest for a little longer, until the full number was reached of both their fellow servants and their brothers who were going to be killed just as they had been.