Apocalypse 6:9-11

And when he had opened the fifth seal, I saw under the altar the souls of them that were slain for the word of God, and for the testimony which they held. And they cried with a loud voice, saying: How long, O Lord (holy and true) dost thou not judge and revenge our blood on them that dwell on the earth? And white robes were given to every one of them one; and it was said to them, that they should rest for a little time, till their fellow servants, and their brethren, who are to be slain, even as they, should be filled up.