The Jataka, Vol. II, tr. by W.H.D. Rouse, , at sacred-texts.com
"Grass and the scum of gruel," etc.--This story the Master told at Jetavana about the Elder Sāriputta.
It once fell out that the Buddha had been spending the rainy season in Sāvatthi, and afterwards had been on alms-pilgrimage. On his return, the inhabitants determined to welcome his home-coming and they made their gifts to the Buddha and his following. They posted the clerk who used to sound the
call for preaching, to distribute the Brethren amongst all comers, according to the number they wished to provide for.
There was one poor old woman, who had prepared one portion. The Brethren were assigned, some to this giver, some to that. At sunrise, the poor woman came to the clerk, and said, "Give a Brother to me!" He answered, "I have already distributed them all; but Elder Sāriputta is still in the monastery, and you may give your portion to him." At this she was delighted, and waited by the gate of Jetavana until the Elder came out. She gave him greeting, took his bowl from his hand, and leading him to her house, offered him a seat.
Many pious families heard a rumour that some old woman had got Sāriputta to sit down at her door. Amongst those who heard it was king Pasenadi the Kosala. He at once sent her food of all sorts, together with a garment and a purse of a thousand pieces, with the request, "Let her who is. entertaining the priest, put on this robe, and spend this money, and thus entertain the Elder." As the king did, so did Anātha-piṇḍika,  the younger Anātha-piṇḍika, the lay sister Visākhā (a great lady),--all sent the same: other families sent one hundred, two hundred or so, as their means allowed. Thus in a single day the old woman got as much as a hundred thousand pieces of money.
Our Elder drank the broth which she gave him, and ate her food, and the rice that she cooked; then he thanked her, and so edified her that she was converted. Then he returned to the monastery.
In the Hall of Truth, the brethren discussed the Elder's goodness. "Friend, the Captain of the Faith has rescued an old housewife from poverty. He has been her mainstay. The food she offered he did not disdain to eat."
The Master entered, and asked what they were talking of now as they sat together. They told him. And he said, "This is not the first time, Brethren, that Sāriputta has been the refuge of this old woman; nor the first time he did not disdain to eat the food she offered. He did the same before." And he told an old-world tale.
It happened once upon a time, when Brahmadatta was king of Benares, that the Bodhisatta was born into a trader's family in the Northern province. Five hundred people of that country, horse-dealers, used to convey horses to Benares, and sell them there.
Now a certain dealer took the road to Benares with five hundred horses for sale. On this road, not far off Benares, there is a town, where had formerly lived a rich merchant. A vast dwelling once was his; but his family had gradually gone down in the world, and only one old woman was left, who lived in the family house. The dealer took up his lodging for a certain hire in that house, and kept his horses hard by.
On that very day, as luck would have it, a thoroughbred mare of his foaled. He tarried two or three days, and then taking his horses with him went off to visit the king. Thereat the old woman asked him for the hire of the house.
"All right, mother, I'll pay you," said he. 
"When you pay me, my son," she said then, "give me this foal, and deduct its value from the hire." The dealer did as she asked and went his way. The woman loved the foal like a son; and she fed him upon parched rice drippings, on broken meats, and grass.
Some time after, the Bodhisatta, on his way with five hundred horses,
took lodging in this house. But the horses scented this highbred foal, that fed on red rice-powder, and not one of them would enter the place. Then said the Bodhisatta to the dame,
"There seems to be some horse in the place, mother?"
"Oh, my son, the only horse there is a young foal which I keep here as tenderly as it were my son!"
"Where is he, mother?"
"Gone out to graze."
"When will he return?"
"Oh, he'll soon come back."
The Bodhisatta kept the horses without, and sat down to wait until the
foal should come in; and soon the foal returned from his walk. When he set eyes on the fine foal with his belly full of rice powder, the Bodhisatta noted his marks, and thought he, "This is a priceless thoroughbred; I must buy him of the old woman."
By this time the foal had entered the house and gone to his own stable. At once all the horses were able to go in too.
There abode the Bodhisatta for a few days, and attended to his horses. Then as he made to go, "Mother," said he to the old woman, "let me buy this foal of you."
"What are you saying! one mustn't sell one's own foster child!"
"What do you give him to eat, mother?"
"Rice boiled, and rice gruel, and parched rice; broken meats and grass; and rice-broth to drink."
"Well, mother, if I get him, I'll feed him on the daintiest of fare;  when he stands, he shall have a cloth awning spread over him; I will give him a carpet to stand on."
"Will you, my son? Then take this child of mine, and go, and may he be happy!"
And the Bodhisatta paid a separate price for the foal's four feet, for his tail and for his head; six purses of a thousand pieces of money he laid down, one for' each; and he caused the dame to robe herself in a new dress, and decked her with ornaments, and set her in front of the foal. And the foal opened his eyes, and looked upon his mother, and shed tears. She stroked his back, and said, "I have received the recompense for what I have done for thee: go, my son!" and then he departed.
Next day the Bodhisatta thought he would make trial of the foal, whether he knew his own power or no. So after preparing common food, he caused red rice gruel to be poured out, presented to him in a bucket. But this he could not swallow; and refused to touch any such food. Then the Bodhisatta to test him, uttered the first verse:
On hearing which, the Foal answered with the two other couplets following:--
"But I am chief of steeds, as you are ware;
Therefore from you I will not take this fare."
 Then answered the Bodhisatta, "I did this to try you; do not be angry"; and he cooked the fine food and offered it to him. When he came to the king's courtyard, he set the five hundred horses on one side, and on the other an embroidered awning, under which he laid a carpet, with a canopy of stuff over it; and here he lodged the foal.
The king coming to inspect the horses asked why this horse was housed apart.
"O king," was the reply, "if this horse be not kept apart, he will let loose these others."
"Is he a beautiful horse?" the king asked.
"Yes, O king."
"Then let me see his paces."
The owner caparisoned him, and mounted on his back. Then he cleared the courtyard of men, and rode the horse about in it. The whole place appeared to be encircled with lines of horses, without a break!
Then said the Bodhisatta, "See my horse's speed, O king!" and let him have his head. Not a man could see him at all! Then he fastened a red leaf upon the horse's flank; and they saw just the leaf. And then he rode him over the surface of a pond in a certain garden of the city. Over he went, and not even the tips of his hoofs were wet. Again, he galloped over lotus leaves,  without even pushing one of them under water.
When his master had thus showed off the steed's magnificent paces, he dismounted, clapped his hands, and held out one, palm upwards. The horse got upon it, and stood on the palm of his master's hand, with his four feet close together. And the Bodhisatta said, "O mighty king! not even the whole circle of the ocean would be space enough for this horse to show off all his skill." The king was so pleased that he gave him the half of his kingdom: the horse he installed as his horse of state, sprinkling him with ceremonial water. Dear was he and precious to the king, and great honour was done him; and his dwelling place was made like the chamber where the king dwelt, all beautiful: the floor was sprinkled with all the four manners of perfumes, the walls were hung with wreaths of flowers and frequent garlands; up in the roof was an awning of cloth spangled with golden stars; it was all like a lovely pavilion round about. A lamp of scented oil burnt always; and in the retiring closet was set a golden jar. His food was always fit for a king. And after he came there,
the lordship over all India came into this king's hand. And the king did good deeds and almsgiving according to the Bodhisatta's admonition, and became destined for paradise.
When the Master had ended this discourse, he declared the Truths, and identified the Birth: (now at the conclusion of the Truths many entered the First Path, or the Second, or the Third:) "At that time the old woman was the same, Sāriputta was the thoroughbred, Ānanda was the king, and the horsedealer was I myself."