Revelation 6:9-11

And when he opened the fifth seal, I saw underneath the altar the souls of them that had been slain for the word of God, and for the testimony which they held: and they cried with a great voice, saying, How long, O Master, the holy and true, dost thou not judge and avenge our blood on them that dwell on the earth? And there was given them to each one a white robe; and it was said unto them, that they should rest yet for a little time, until their fellow-servants also and their brethren, who should be killed even as they were, should have fulfilled their course.