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 other servants and believers were killed, as they had been. 1
References for Revelation 6:11