The Jataka, Vol. II, tr. by W.H.D. Rouse, , at sacred-texts.com
 "Fine are the fish," etc.--This story the Master told while dwelling at Jetavana, about two young Brethren.
These two young fellows, we are told, belonged to a good family of Sāvatthi, and had embraced the faith. But they, not realising the impurity of the body 1, sang the praises of their beauty, and went about bragging of it.
One day they fell Into a dispute on this point: "You're handsome, but so am I," said each of them; then, spying an aged Elder sitting not far away, they agreed that he was likely to know whether they were beautiful or not. Then they approached him with the question, "Sir, which of us is beautiful?" The Elder replied, "Friends, I am more beautiful than either of you." At this the young men reviled him, and went off, grumbling that he told them something they did not ask, but would not tell them what they did.
The Brotherhood became aware of this event; and one day, when they were all together in the Hall of Truth, they began talking about it. "Friend, how the old Elder shamed those two young fellows whose heads were full of their own beauty!" The Master came in, and asked what they were talking of now as they sat together. They told him. He rejoined, "This, is not the only time, Brethren, that our friends were full of the praises of their own beauty. In olden times they used to go about boasting of it as they do now." And then he told them an old-world tale.
Once upon a time, during the reign of Brahmadatta, king of Benares, the Bodhisatta became a tree sprite on the bank of the Ganges. At the point where Ganges and Jumna meet, two fish met together, one from the Ganges and one from the Jumna. "I am beautiful!" said one, "and so are you!" and then they fell to quarrelling about their beauty. Not far from the Ganges they saw a Tortoise lying on the bank. "Yon fellow shall decide whether or no we are beautiful!" said they; and they went up to him. "Which of us is beautiful, friend Tortoise," they asked, "the Ganges fish or the Jumna fish? "The Tortoise answered, "The Ganges fish is beautiful, and the Jumna fish is beautiful: but I am more beautiful than you both." And to explain it, he uttered the first verse:--
When the fish heard this, they cried, "Ah, you rascally Tortoise! you won't answer our question, but you answer another one!" and they repeated the second verse:
When this discourse was concluded, the Master identified the Birth:--"In those days the young Brothers were the two fish, the old man was the tortoise, and I was the tree-sprite who saw the whole thing from the Ganges bank."
104:1 Reading an-anuyuñjitvā.