And when the Lamb broke the fifth seal, I saw under the altar the souls of all who had been martyred for the word of God and for being faithful in their witness.
They called loudly to the Lord and said, "O Sovereign Lord, holy and true, how long will it be before you judge the people who belong to this world for what they have done to us? When will you avenge our blood against these people?"
Then a white robe was given to each of them. And they were told to rest a little longer until the full number of their brothers and sisters -- their fellow servants of Jesus -- had been martyred.
References for Revelation 6:11