In Mauville City, there is a house south of the Bike Shop. In the house live a person called the "Rock Master". Talk to him, and he will give you TM 06, Rock Smash.
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HM06 Rock Smash
he means go up to a rock and use rock smash and maybe a Pokemon will jump out.
rock smash isn't in Pokemon red version
Victory Road (use rock smash)
A wide variety of Pokemon can use Rock Smash. Most Fighting, Rock and Ground type Pokemon can learn the move Rock Smash from the TM or HM depending on the Pokemon version.