Reference for Avodah Zarah 93:19

לעולם למיא קא סגיד והכי קמבעיא ליה למיא דקמיה קא סגיד וקמאי קמאי אזדו או דלמא לדברונא דמיא קא סגיד

Rather we assume that he worshipped the water; and this is the point of his question: Did he worship the water which was in front of him and that water has flowed away, or perhaps he worshipped the flow of the water?

Jastrow