There are several versions of the song, but the most common lyrics are:

Hush, little baby, don't say a word,
Mama's gonna buy you a mockingbird.
And if that mockingbird don't sing,
Mama's gonna buy you a diamond ring.
And if that diamond ring turns brass,
Mama's gonna buy you a looking glass.
And if that looking glass gets broke,
Mama's gonna buy you a billy goat,
And if that billy goat doesn't pull,
Mama's gonna buy you a cart and bull.
And if that cart and bull turn over,
Mama's gonna buy you a dog named Rover.
And if that dog named Rover won't bark,
Mama's gonna buy you a horse and cart.
And if that horse and cart fall down,
Well you'll still be the sweetest baby in town.

from Wikipedia