Revelation 6:9 in the King James Version
And when he had opened the fifth seal, I saw under the altar the souls of them that were slain for the word of God, and for the testimony which they held:
Context
8 And behold, a pale horse, and the name of he who sat on it was Death. Hades followed with him. Authority over one fourth of the earth, to kill with the sword, with famine, with death, and by the wild animals of the earth was given to him.
9 When he opened the fifth seal, I saw underneath the altar the souls of those who had been killed for the Word of God, and for the testimony of the Lamb which they had.
10 They cried with a loud voice, saying, “How long, Master, the holy and true, until you judge and avenge our blood on those who dwell on the earth?”