Who sang it?

[Joy]

Sing all ye joyful, now sing all together!
The wind's in the tree-top, the wind's in the heather;
The stars are in blossom, the moon is in flower,
And bright are the windows of Night in her tower.
Dance all ye joyful, now dance all together!
Soft is the grass, and let foot be like feather!
The river is silver, the shadows are fleeting;
Merry is May-time, and merry our meeting.
Sing we now softly, and dreams let us weave him!
Wind him is slumber and there let us leave him!
The wanderer sleepeth. Now soft be his pillow!
Lullaby! Lullaby! Alder and Willow!
Sigh no more Pine, till the wind of the morn!
Fall Moon! Dark be the land!
Hush! Hush! Oak, Ash and Thorn!
Hushed be all water, till dawn is at hand!

It is sung in Rivendell in the "Hobbit" The chapter The Last Stage.