Revelation 6:9
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. 10 They cried out with a loud voice, “How long, Sovereign Master, holy and true, before you judge those who live on the earth and avenge our blood?” 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.
Sifre Numbers 139
Halakhic Midrash(Bamidbar 27:16) "Let the L-rd, the G-d of the spirits of all flesh, etc.": Scripture hereby apprises us that all the spirits issue only from Him. R. Eliezer the son of R. Yossi Haglili says: Let this "sign" always be in your hand: that so long as a man is alive his soul is reposited in the hand of its Owner, as it is written (Iyyov 12:10) "… that in His hand is the spirit of all living things. When he dies, it is reposited in the otzar ("the treasury,") as it is written (I Samuel 25:29) "and may the soul of my master be bound up in the bond of life." I might think, (the soul of) both the righteous and the wicked; it is, therefore, written (Ibid.) "and may He sling out the soul of your foes (as) in the hollow of a sling." "a man over the congregation": This is Joshua, as it is written (Psalms 78:25) "the bread of the mighty (i.e., manna) did a man eat." And why did Scripture not specify (that it was Joshua)? So as not to stir up controversy (over the appointment of Joshua) among his sons and the sons of his brother (Aaron, i.e., Elazar and Ithamar.)"
Notes and References
Mandel, Paul The Sacrifice of the Souls of the Righteous upon the Heavenly Altar: Transformations of Apocalyptic Traditions in Medieval Ashkenaz (pp. 49-72) Liverpool University Press, 2018