פירוש על עבודה זרה 117:3

Earlier, on daf 46 there was a discussion about boulders that had become detached from a mountain on their own force, without human intervention. Can such boulders become prohibited if worshiped? Water that becomes detached by a wave is similar—a natural feature that becomes detached from its source on its own. So if R. Yohanan holds that it is prohibited, he must be the one who held that boulders detached from the ground are prohibited.
The Talmud again refines the case—the water worshiped here was not detached simply by a wave. Rather, a non-Jew struck the water, then worshiped the detached water. Can a Jew now use such water? R. Yohanan would say yes, but this does not mean that he is the one that holds that the boulders are permitted.