The Jataka, Volume I, tr. by Robert Chalmers, , at sacred-texts.com
"Though prostrate now."--This story was told by the Master while at Jetavana about a Brother who gave up persevering. For it was then that the Master addressed that Brother and said, "Brethren, in bygone days the wise and good persevered even amid hostile surroundings, and, even when they were wounded, still did not give in." And, so saying, he told this story of the past.
Once on a time when Brahmadatta was reigning in Benares, the Bodhisatta came to life as a thoroughbred Sindh horse and was made the king's destrier, surrounded by all pomp and state. He was fed on exquisite three-year old rice, which was always served up to him in a golden dish worth a hundred thousand pieces of money; and the ground of his stall was perfumed with the four odours. Round his stall were hung crimson curtains, while overhead was a canopy studded with stars of gold. On the walls were festooned wreaths and garlands of fragrant flowers; and a lamp fed with scented oil was always burning there.
Now all the kings round coveted the kingdom of Benares. Once seven kings encompassed Benares, and sent a missive to the king, saying, "Either yield up your kingdom to us or give battle." Assembling his ministers, the king of Benares laid the matter before them, and asked them what he was to do. Said they, "You ought not to go out to do battle in person, sire, in the first instance.  Despatch such and such a knight out first to fight them; and later on, if he fails, we will decide what to do."
Then the king sent for that knight and said to him, "Can you fight the seven kings, my dear knight?" Said he, "Give me but your noble destrier, and then I could fight not seven kings only, but all the kings in India." "My dear knight, take my destrier or any other horse you please, and do battle." "Very good, my sovereign lord," said the knight; and with a bow he passed down from the upper chambers of the palace. Then he had the noble destrier led out and sheathed in mail, arming himself too cap-à-pie,
and girding on his sword. Mounted on his noble steed he passed out of the city-gate, and with a lightning charge broke down the first camp, taking one king alive and bringing him back a prisoner to the soldiers' custody. Returning to the field, he broke down the second and the third camps, and so on until he captured alive five kings. The sixth camp he had just broken down, and had captured the sixth king, when his destrier received a wound, which streamed with blood and caused the noble animal sharp pain. Perceiving that the horse was wounded, the knight made it lie down at the king's gate, loosened its mail, and set about arming another horse. As the Bodhisatta lay at full length on his side, he opened his eyes, and gathered what the knight was doing. "My rider," thought he to himself, "is arming another horse. That other horse will never be able to break down the seventh camp and capture the seventh king; he will lose all that I have accomplished. This peerless knight will be slain; and the king, too, will fall into the hands of the foe. I alone, and no other horse, can break down that seventh camp and capture the seventh king." So, as he lay there, he called to the knight, and said, "Sir knight, there is no horse but I who can break down the seventh camp and capture the seventh king. I will not throw away what I have already done; only have me set upon my feet and clad again in my armour." And so saying, he repeated this stanza:--
The knight had the Bodhisatta set upon his feet, bound up his wound, and armed him again in proof. Mounted on the destrier, he broke down the seventh camp, and brought back alive the seventh king, whom he handed over to the custody of the soldiers. They led the Bodhisatta too up to the king's gate, and the king came out to look upon him. Then said the Great Being to the king, "Great king, slay not these seven kings; bind them by an oath, and let them go. Let the knight enjoy all the honour due to us both, for it is not right that a warrior who has presented you with seven captive kings should be brought low. And as for yourself, exercise charity, keep the Commandments, and rule your kingdom in righteousness and justice." When the Bodhisatta had thus exhorted the king, they took off his mail; but when they were taking it off piecemeal, he passed away.
The king had the body burned with all respect, and bestowed great honour on the knight, and sent the seven kings to their homes after exacting from each an oath never to war against him any more. And he ruled his kingdom in righteousness and justice, passing away when his life closed to fare thereafter according to his deserts.
Then the Master said, "Thus, Brethren, in bygone days the wise and good persevered even amid hostile surroundings, and, even when wounded so grievously, still did not give in. Whereas you who have devoted yourself to so saving a doctrine,--how comes it that you give up persevering?" After which, he preached the Four Truths, at the close whereof the faint-hearted Brother won Arahatship. His lesson ended, the Master  shewed the connexion, and identified the Birth by saying, "Ānanda was the king of those days, Sāriputta the knight, and I myself the thorough-bred Sindh horse."