Sweetie Belle's, err, 'lullaby' from episode 17.

Fluttershy's Version: http://www.youtube.com/watch?v=EHwE_BVsnMQ

Lyrics:

Hush now! Quiet now! 
It's time to lay your sleepy head! 
Said hush now! Quiet now! 
It's time to go to bed! 

Driftin' (driftin') off to sleep! 
The exciting day behind you! 
Driftin' (driftin') off to sleep! 
Let the joy of dreamland find you! 

Hush now! Quiet now! 
Lay your sleepy head! 
Said hush now! Quiet now! 
This's time to go to bed! 
OW!!! 

Lyrics courtesy of the MLP Wiki
http://mlp.wikia.com/