The Jataka, Volume I, tr. by Robert Chalmers, , at sacred-texts.com
"No morsel can he eat."--This story was told by the Master while at Jetavana, about a lay-disciple and an aged Elder. 
Tradition says that there were in Sāvatthi two friends, of whom one joined the Brotherhood but used to go every day to the other's house, where his friend used to give him an alms of food and make a meal himself, and then accompany him back to the Monastery, where he sat talking all the livelong day till the sun went down, when he went back to town. And his friend the Brother used to escort him on his homeward way, going as far as the city-gates before turning back.
The intimacy of these two became known among the Brethren, who were sitting one day in the Hall of Truth, talking about the intimacy which existed between the pair, when the Master, entering the Hall, asked what was the subject of their talk; and the Brethren told him.
"Not only now, Brethren, are these two intimate with one another," said the Master; "they were intimate in bygone days as well." And, so saying, he told this story of the past.
Once on a time when Brahmadatta was reigning in Benares, the Bodhisatta became his minister. In those days there was a dog which used to go to the stall of the elephant of state, and eat the gobbets of rice which fell where the elephant fed. Haunting the place for the food's sake,
the dog grew very friendly with the elephant, and at last would never eat except with him. And neither could get on without the other. The dog used to disport himself by swinging backwards and forwards on the elephant's trunk. Now one day a villager bought the dog of the mahout and took the dog home with him. Thenceforward the elephant, missing the dog, refused either to eat or drink or take his bath; and the king was told of it. His majesty despatched the Bodhisatta to find out why the elephant behaved like this. Proceeding to the elephant-house, the Bodhisatta, seeing how sad the elephant was, said to himself, "He has got no bodily ailment; he must have formed an ardent friendship, and is sorrowing at the loss of his friend." So he asked whether the elephant had become friends with anyone.
"Yes, my lord," was the answer; "there's a very warm friendship between him and a dog." "Where is that dog now?" "A man took it off." "Do you happen to know where that man lives?" "No, my lord." The Bodhisatta went to the king and said, "There is nothing the matter with the elephant, sire; but he was very friendly with a dog,  and it is missing his friend which has made him refuse to eat, I imagine." And so saying, he repeated this stanza:
"Well," said the king on hearing this; "what is to be done now, sage?" "Let proclamation be made by beat of drum, your majesty, to the effect that a man is reported to have carried off a dog of which the elephant of state was fond, and that the man in whose house that dog shall be found, shall pay such and such a penalty." The king acted on this advice; and the man, when he came to hear of it, promptly let the dog loose. Away ran the dog at once, and made his way to the elephant. The elephant took the dog up in his trunk, and placed it on his head, and wept and cried, and, again setting the dog on the ground, saw the dog eat first and then took his own food.
"Even the minds of animals are known to him," said the king, and he loaded the Bodhisatta with honours.
Thus the Master ended his lesson to show that the two were intimate in bygone days as well as at that date. This done, he unfolded the Four Truths. (This unfolding of the Four Truths forms part of all the other Jātakas; but we shall only mention it where it is expressly mentioned that it was blessed unto fruit.) Then he shewed the connexion, and identified the Birth by saying, "The lay-disciple was the dog of those days, the aged Elder was the elephant, and I myself the wise minister."