The Jataka, Vol. II, tr. by W.H.D. Rouse, , at sacred-texts.com
"When I was in their cage," etc.--This story the Master told while dwelling in the Bamboo-grove, about Ajātasattu.
Mahā-Kosala, the king of Kosala's father, when he married his daughter to king Bimbisāra, had given her a village in Kāsi for bath-money. After Ajātasattu murdered Bimbisāra, his father, the queen very soon died of love for him. Even after his mother's death, Ajātasattu still enjoyed the revenues of this village. But the king of Kosala determined that no parricide should have a village which was his by right of inheritance, and made war upon him. Some-times the uncle got the best of it, and sometimes the nephew. And when Ajātasattu was victor, he raised his banner and marched through the country back to his capital in triumph; but when he lost, all downcast he returned without letting any one know.
It happened on a day that the Brethren sat talking about it in the Hall of Truth. "Friend"--so one would say--"Ajātasattu is delighted when he beats his uncle, and when he loses he is cast down." The Master, entering the Hall, asked what they were discussing this time;  and they told him. He said, "Brethren, this is not the first time that the man has been happy when he conquered, and miserable when he did not." And he told them an old-world tale.
Once upon a time, when Brahmadatta was king of Benares, the Bodhisatta became a Green Frog. At the time people set wicker cages in all pits and holes of the rivers, to catch fish withal. In one cage were a large number of fish. And a Water-snake, eating fish, went into the trap himself. A number of the fish thronging together fell to biting him, until he was covered with blood. Seeing no help for it, in fear of his life he slipped out of the mouth of the cage, and lay down full of pain on the edge of the water. At the same moment, the Green Frog took a leap and fell into the mouth of the trap. The Snake, not knowing to whom he could appeal, asked the Frog that he saw there in the trap--"Friend Frog, are you pleased with the behaviour of yonder Fish?" and he uttered the first stanza:--
Then the Frog answered him, "Yes, friend Snake, it does: why not? if you eat fish which get into your demesne,  the fish eat you when you get into theirs. In his own place, and district, and feeding ground no one is weak." So saying, he uttered the second stanza:
The Bodhisatta having pronounced his opinion, all the fish observing the Snake's weakness, cried, "Let us seize our foe!" and came out of the cage, and did him to death then and there, and then departed.
When the Master had ended this discourse, he identified the Birth:--"Ajātasattu was the Water-snake, and the Green Frog was I."