Hey dol! merry dol! ring a dong dillo!
From Tolkien Gateway
Hey dol! Merry dol! Ring a Dong Dilo! is one of the songs Tom Bombadil sung in The lord of the rings.
The Poem
Hey dol! merry dol! ring a dong dillo!
Ring a dong! hop along! fal lal the willow!
Tom Bom, jolly Tom, Tom Bombadillo!