The Jataka, Volume I, tr. by Robert Chalmers, , at sacred-texts.com
"With heavy loads."--This story was told by the Master while at Jetavana, about the Double Miracle, which, together with the Descent from Heaven, will be related in the Thirteenth Book, in the Sarabhamiga-jātaka 1.
After he had performed the Double Miracle and had made a stay in Heaven, the All-knowing Buddha descended at the city of Saṁkassa on the day of the Great Pavāraṇā 2 Festival, and thence passed with a large following to Jetavana.
Gathering together in the Hall of Truth, the Brethren sat praising the virtues of the Master, saying, "Sirs, peerless is the Buddha; none may bear the yoke borne by the Buddha. The Six teachers, though they protested so often that they, and they only, would perform miracles, yet not a single miracle did they work. O! how peerless is the Master!"
Entering the Hall and asking the theme which the Brethren were discussing in conclave , the Master was informed that their theme was no other than his own virtues. "Brethren," said the Master, "who shall now bear the yoke borne by me? Even in bygone days, when I came to life as an animal, I was unmatched." 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 bull. And while he was still a young calf, his owners, who had been lodging with an old woman, made him over to her in settlement of their reckoning. She reared him like her own child, feeding him on rice-gruel and rice and on other good cheer. The name he became known by was "Granny's Blackie." Growing up, he used to range about with the other cattle of the village, and was as black as jet. The village urchins used to catch hold of his horns and ears and dewlaps, and have a ride; or they would hold on to his tail in play, and mount on his back.
One day he thought to himself, "My mother is very poor; she has painfully reared me, as if I were her own child. What if I were to earn some money to ease her hard lot?" Thenceforth he was always looking out for a job. Now, one day a young merchant at the head of a caravan came with five hundred waggons to a ford the bottom of which was so rough that his oxen could not pull the waggons through. And even when he took out the five hundred pairs of oxen and yoked the lot together to form one team, they could not get a single cart by itself across the river. Close
by that ford the Bodhisatta was about with the other cattle of the village, And the young merchant, being a judge of cattle, ran his eye over the herd to see whether among them there was a thorough-bred bull who could pull the waggons across. When his eye fell on the Bodhisatta, he felt sure he would do; and, to find out the Bodhisatta's owner, he said to the herdsmen, "Who owns this animal? If I could yoke him on and get my waggons across, I would pay for his services." Said they, "Take him and harness him, then; he has got no master hereabouts."
But when the young merchant slipped a cord  through the Bodhisatta's nose and tried to lead him off, the bull would not budge. For, we are told, the Bodhisatta would not go till his pay was fixed. Understanding his meaning, the merchant said, "Master, if you will pull these five hundred waggons across, I will pay you two coins per cart, or a thousand coins in all."
It now required no force to get the Bodhisatta to come. Away he went, and the men harnessed him to the carts. The first he dragged over with a single pull, and landed it high and dry; and in like manner he dealt with the whole string of waggons.
The young merchant tied round the Bodhisatta's neck a bundle containing five hundred coins, or at the rate of only one for each cart. Thought the Bodhisatta to himself, "This fellow is not paying we according to contract! I won't let him move on!" So he stood across the path of the foremost waggon and blocked the way. And try as they would, they could not get him out of the way. "I suppose he knows I've paid him short," thought the merchant; and he wrapped up a thousand coins in a bundle, which he tied round the Bodhisatta's neck, saying, "Here's your pay for pulling the waggons across." And away went the Bodhisatta with the thousand pieces of money to his "mother."
"What's that round the neck of Granny's Blackie?" cried the children of the village, running up to him. But the Bodhisatta made at them from afar and made them scamper off, so that he reached his "mother" all right. Not but what he appeared fagged out, with his eyes bloodshot, from dragging all those five hundred waggons over the river. The pious woman, finding a thousand pieces of money round his neck, cried out, "Where did you get this, my child?" Learning from the herdsmen what had happened, she exclaimed, "Have I any wish to live on your earnings, my child? Why did you go through all this fatigue?" So saying, she washed the Bodhisatta with warm water and rubbed him all over with oil; she gave him drink and regaled him with due victuals. And when her life closed, she passed away, with the Bodhisatta, to fare according to her deserts.
When he had ended this lesson to shew that the Buddha was unmatched in the past as then, he shewed the connexion by uttering, as Buddha, this stanza:--
After his lesson to shew that only 'Blackie' could draw the load, he shewed the connexion, and identified the Birth by saying, "Uppala-Vaṇṇā was the old woman of those days, and I myself 'Granny's Blackie.'"
73:1 No. 483.
73:2 The festival at the end of the rainy season (Mahāvagga IV. 1).