Warden’s Quest is required to obtain HM06 Rock Smash.
To start the quest, speak to Warden in Fuchsia City. He’s in the building with the red roof on the south side of the city to the right of the Pokemon Center. He will give you a quest which requires you to find his teeth.
Once you grab his Gold Teeth, return them to Warden and he will give you HM06 Rock Smash.