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 testimony.
They shouted to the Lord and said, “O Sovereign Lord, holy and true, how long before you judge the people who belong to this world and avenge our blood for what they have done to us?”
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 who were to be martyred—had joined them.
References for Revelation 6:11