It is in the house to the right of the PokeMart in Mauville city. You will walk inside the house and talk to a man that looks a bit like a scientist, then he will say something like, "Woohoo! Your Pokemon look pretty strong! Here, I'd like you to have this HM Rock Smash."
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HM06 Rock Smash
Victory Road (use rock smash)
you can get strength HM from using rock smash on the rock that seperates the couple in the village across from marvile which you get rock smash from.
he means go up to a rock and use rock smash and maybe a Pokemon will jump out.
Nincada is a fantastic Pokemon to use these moves on. Not only can it learn Flash and Rock Smash, but also Cut, Dig, and Secret Power which are all moves that are required later on.