Revelation 6:9

6:9 Now when the Lamb opened the fifth seal, I saw under the altar the souls of those who had been violently killed because of the word of God and because of the testimony they had given.

Revelation 6:11

6:11 Each of them was given a long white robe and they were told to rest for a little longer, until the full number was reached of both their fellow servants and their brothers who were going to be killed just as they had been.