The Fifth Seal—Martyrs
9 When the Lamb broke the fifth seal, I saw underneath the altar the souls of those who had been slain because of the word of God, and because of the testimony which they had maintained; 10 and they cried out with a loud voice, saying, “How long, O Lord, holy and true, will You refrain from judging and avenging our blood on those who dwell on the earth?” 11 And there was given to each of them a white robe; and they were told that they should rest for a little while longer, until the number of their fellow servants and their brethren who were to be killed even as they had been, would be completed also.
Excellent reference from the Book of Revelation.