Rabbi Akiva said: Every day, an angel stands in the middle of the firmament and begins and says, "The Lord is the King," and the whole celestial retinue answers after it; until it reaches, 'Barchu' (the prayer leader's invitation to the congregation to bless God). Once it reaches, "Barchu," there is a chaya (a lofty angel), whose name is Israel - with "My people is Mine" imprinted on its forehead - who stands in the middle of the firmament and and says, "Bless (Barchu) the Lord who is blessed!"
And all of the ministers on high answer after it, "Blessed is the Lord who is blessed forever and ever!" Before it finishes its words, the ofanim (another group of angels) shout and shake, and shake up the whole world, saying, "Blessed is the glory of the Lord from His place!" And that chaya stands in the middle of the firmament until all of the ministers on high and the officers; and the divisions and all of the hosts shake. And each one says in its place to the chaya, "Hear O Israel, the Lord is our God, the Lord is One" (Deuteronomy 6:4).