You go to an island and you talk to an old man (wearing blue). He will say that he doesn't need it and he will give you this hm.
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The HM06 which contains Rock Smash is given by an old man on One Island in the ember spa.
It will be at the top of Mt. Ember on One Island. You will need Rock Smash and Strength to get there.
Rock Smash can be found on Island 1, in the little spa area on Kindle Road.
Surf to get there and Rock Smash inside for the maze (Fire Red)
rock smash isn't in Pokemon red version