If you already have the hm rock smash, make sure you teach it to someone. Go to the Rusturf tuneel cave thing and you should be able to find two rocks. Use rock smash on one of them and a man will give you the hm
strength after the chamber
H.M
by using the hm strength
by wining a certain gym battle
you recieve the hm waterfall after you have deafeated the elite 4
the hm move strength is found in the cave Rusturf tunnel
Using Rock Smash move, break the rocks inside Rusturf Tunnel and the guy there will give you HM Strength.
If you have beaten Mauville Gym and have gotten HM ROCK SMASH, go to RUSTURF TUNNEL and clear out the rocks. The digger will grant you HM STRENGTH.
This HM is exclusive to ruby, sapphire and emerald versions. You receive it from steven after getting the 7th badge.
A Pokémon needs the HM move of Strength in order to move the big boulders. You can find the Strength HM in the Rusturf Tunnel.
You can find a Fire Stone in the Fiery Path after you get the HM for Strength.
In rusturf tunnel.