An Historical Summary of Angelic Hierarchies from Part VII: The “Seven” Thrones in In Operibus Sigillo Dei Aemeth by David Richard Jones.
“The earliest known reference to Thrones as an angelic order is found in the text of The Testament of Levi, in the pseudopigraphic collection referred to as The Testament of the Twelve Patriarchs.
And in the highest of all dwelleth the Great Glory, far above all holiness. In the heaven next to it are the archangels, who minister and make propitiation to the Lord for all the sins of ignorance of the righteous; Offering to the Lord a sweet smelling savor, a reasonable and a bloodless offering. And in the heaven below this are the angels who bear answers to the angels of the presence of the Lord. And in the heaven next to this are thrones and dominions, in which always they offer praise to God. When, therefore, the Lord looketh upon us, all of us are shaken; yea, the heavens and the earth, and the abysses are shaken at the presence of His majesty. But the sons of men, having no perception of these things, sin and provoke the Most High.
Testament of Levi I: 21–27 (trans. R.H. Charles)” [via]