Tennyson's Idylls of the King

From The Holy Grail, as King Arthur addresses those few returned from the errant quest, of whom only a couple actually saw the Grail:

"For every fiery prophet in old times,
And all the sacred madness of the bard,
When God made music through them, could but speak
His music by the framework and the chord;
As ye saw it ye have spoken truth."