From the Super Mario Wiki
A Sleepy Bomb is a status-altering item found in Super Mario RPG: Legend of the Seven Stars. Mario can buy a Sleepy Bomb in Tadpole Pond for one Frog Coin. They can also be obtained by defeating certain enemies in battle. When used in a battle, all enemies will fall asleep (except enemies immune to the status ailment, as well as some stronger enemies and Bosses).