Fluttershy's Lullaby from episode 17.

Sweetie Belle's Version: http://www.youtube.com/watch?v=SA34YhfDMIw

Lyrics:

Hush now, quiet now 
It's time to lay your sleepy head 
Hush now, quiet now 
It's time to go to bed 

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