Gaṅgeyya Jātaka (No.205)
Two fish, one from Gaṅgā, the other from Yamunā, once met at the confluence of the rivers and disputed as to their relative beauty. They appealed to a turtle who was there for a decision; he said they were both beautiful, but he himself was more beautiful than either.
The story was told in reference to two monks who bragged of their good looks and quarrelled about them. They appealed to an older monk, who gave the same answer as the turtle of the story. J.ii.151 f.