It is from the "Ballad of the White Horse" and in Book 3 "The Harp of Alfred." Any links to the poem just give the whole thing, but it's down near the end. This is the whole stanza, and the two surrounding:

He heaved the head of the harp on high
And swept the framework barred,
And his stroke had all the rattle and spark
Of horses flying hard.

"When God put man in a garden
He girt him with a sword,
And sent him forth a free knight
That might betray his lord;

"He brake Him and betrayed Him,
And fast and far he fell,
Till you and I may stretch our necks
And burn our beards in hell.