The Jataka, Vol. II, tr. by W.H.D. Rouse, , at sacred-texts.com
 "The wolf who takes," etc.--This story the Master told at Jetavana, about old friendship. The circumstances were the same in detail as in the Vinaya 1; this is an abstract of them. The reverend Upasena, a two-years' man, visited
the Master along with a first year's man who lived in the same monastery; the Master rebuked him, and he retired. Having acquired spiritual insight, and attained to sainthood, having got contentment and kindred virtues, having undertaken the Thirteen Practices of a Recluse, and taught them to his fellows, while the Blessed One was secluded for three months, he with his brethren, having accepted the blame first given for wrong speech and nonconformity, received in the second instance approval, in the words, "Henceforth, let any brothers visit me when they will, provided they follow the Thirteen Practices of a Recluse." Thus encouraged, he returned and told it to the Brethren. After that, the brothers followed these practices before coming to visit the Master; then, when ho had come out from his seclusion, they would throw away their old rags and put on clean garments. As the Master with all the body of the Brethren went round to inspect the rooms,  he noticed these rags lying about, and asked what they were. When they told him, he said, "Brethren, the practice undertaken by these brothers is short-lived, like the wolf's holy day service"; and he told them an old-world tale.
Once upon a time, when Brahmadatta reigned king in Benares, the Bodhisatta came to life as Sakka, king of the gods. At that time a Wolf lived on a rock by the Ganges bank. The winter floods came up and surrounded the rock. There he lay upon the rock, with no food and no way of getting it. The water rose and rose, and the wolf pondered: "No food here, and no way to get it. Here I lie, with nothing to do. I may as well keep a sabbath feast." Thus resolved to keep a sabbath, as he lay he solemnly resolved to keep the religious precepts. Sakka in his meditations perceived the wolf's weak resolve. Thought he, "I'll plague that wolf"; and taking the shape of a wild goat, he stood near, and let the wolf see him.
"I'll keep Sabbath another day!" thought the Wolf, as he spied him; up he got, and leapt at the creature. But the goat jumped about so that the Wolf could not catch him. When our Wolf saw that he could not catch him, he came to a standstill, and went back, thinking to himself as he lay down again, "Well, my Sabbath is not broken after all."
Then Sakka, by his divine power, hovered above in the air; said he, "What have such as you, all unstable, to do with keeping a Sabbath? You didn't know that I was Sakka, and wanted a meal of goat's-flesh!" and thus plaguing and rebuking him, he returned to the world of the gods.
"When Sakka learnt what he resolved to do,
He made himself a goat to outward view.
Then the blood-bibber leaped to seize his prey,
His vow forgot, his virtue cast away. p. 308
 "Even so some persons in this world of ours,
That make resolves which are beyond their powers,
Swerve from their purpose, as the wolf did here
As soon as he beheld the goat appear."
When the Master had ended this discourse, he identified the Birth as follows: "At that time I myself was Sakka."
END OF THE THIRD BOOK.
306:1 Mahāvagga, i. 31. 3 foll. (trans. in S. B. E., i. p. 175); Folk-Lore Journal, 3. 359; Morris, Contemp. Rev. xxix. 739.