Hush, little baby is an American lullaby - that is, a folk song designed to help children fall asleep. As with most folk songs, the original author is unknown, and many variations of the lyrics exist, including some that have been incorporated into copyrighted songs. A popular variant goes:
Hush, little baby, don't say a word / Mama's gonna buy you a mockin'bird
If that mockin'bird don't sing / Mama's gonna buy you a diamond ring
If that diamond ring turns brass, / Mama's gonna buy you a looking glass
If that looking glass gets broke / Mama's gonna buy you a billy goat
If that billy goat don't pull, / Mama's gonna buy you a cart and mule
If that cart and mule turn over, / Mama's gonna buy you a dog named Rover
If that dog named Rover won't bark, / Mama's gonna buy you a horse and cart
If that horse and cart fall down, / Then you'll be the sweetest little baby in town