Gadrabhapañha Jātaka (No.111)
One of the problems set by King Vedeha to Mahosadha, in order to test him, at the instigation of the king’s ministers. The king sent word to Mahosadha that while he was on his way to see him his horse had broken its leg; would Mahosadha therefore, send him a more excellent horse? Mahosadha, understanding the significance of the message, went to the palace, sending his father on before him. When Mahosadha entered he found his father seated, but, as had been prearranged, he obliged his father to get up and offer him his seat. On this there was a great uproar, all saying that he had slighted his father. However, Mahosadha convinced the king that he was a more excellent man than his father, giving proof of this by producing before the king an ass which he had brought with him, and making the king acknowledge that a colt born of the ass through a Sindh mare would be far more valuable than the colt’s sire (J.vi.342 f).
The story is given as a separate Jātaka (No.111), J.i.424, but the Commentary there just refers to the Umaṅga Jātaka.