Revelation 6:9-11

Then the Lamb broke open the fifth seal. I saw underneath the altar the souls of those who had been killed because they had proclaimed God's word and had been faithful in their witnessing. They shouted in a loud voice, “Almighty Lord, holy and true! How long will it be until you judge the people on earth and punish them for killing us?” Each of them was given a white robe, and they were told to rest a little while longer, until the complete number of their fellow-servants and fellow-Christians had been killed, as they had been.