Cyril of Jerusalem

We make mention also of the seraphim, whom Isaiah in the Holy Spirit saw standing around the throne of God, and with two of their wings veiling their face, and with two their feet, while with two they flew, crying, "Holy, holy, holy is the Lord of hosts." For the reason of our reciting this confession of God, delivered down to us from the seraphim, is this, that so we may be partakers with the hosts of the world above in their hymn of praise.