They had sung their addition to the Trisagion on a Sunday in the Church of St. Michael within the palace.
"The Formation of Christendom, Volume VI" by Thomas W. (Thomas William) Allies
The name is often applied to the Trisagion (tersanctus), or "Holy, Holy, Holy," the scriptural basis of which is found in Isaiah vi.
"Encyclopaedia Britannica, 11th Edition, Volume 8, Slice 6" by Various
The Trisagion rings, and has need to ring, for ever down the Church.
"The Book of Isaiah, Volume I (of 2)" by George Adam Smith
RELIGIOUS RIOTS ABOUT THE TRISAGION.
"Curiosities of Christian History" by Croake James
Why thus, apart,--the swift-winged herald spake,--
Sit ye with silent lips and unstrung lyres
While the trisagion's blending chords awake
In shouts of joy from all the heavenly choirs?
"Homesick In Heaven" by Oliver Wendell Holmes