Sankhara on a wave, a wood, and a rock?

- Purpose: I want to know which sutta is this story coming from. - Background: I remember that when I joined the 10-day Vipassana course held by S.N.Goenka, he said, You can carve your Sankhara like an ocean wave. If you keep carving the Sankhara, eventually, it becomes stronger, like carving it on wood. Someday, it becomes like carving it on a rock. Something like this. It might be incorrect. - Comment: Does anyone know it?