From The Fellowship of the Ring, “In The House of Tom Bombadil”. Frodo sang the song to Goldberry when he first met her.

O slender as a willow-want! O clearer than clear water!
O reed by the living pool! Fair river-daughter!
O spring-time and summer-time, and spring again after!
O wind on the waterfall, and the leaves’ laughter!

Provided by: NenyaGold