When He opened the fifth seal, I saw under the altar the souls of those slaughtered because of God's word and the testimony they had.
References for Revelation 6:9
They cried out with a loud voice: "O Lord, holy and true, how long until You judge and avenge our blood from those who live on the earth?"
References for Revelation 6:10
So 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 slaves and their brothers, who were going to be killed just as they had been, would be completed.