Ekapaṇṇa Jātaka (No. 149)
The Bodhisatta was once a brahmin ascetic of great psychic power (iddhi) and dwelt in the Himalayas. One day he entered Benares and took up his residence in the royal park. The king, pleased with his demeanour, invited him into the palace and asked him to spend the rainy season in the park. The king had an ill-natured son, named Duṭṭhakumāra, and despairing of ever being able to reform him, handed him over, as a last resort, to the ascetic. One day, when the ascetic was walking about in the garden with the prince, he asked him to taste one (eka) leaf (paṇṇa) of a young Nimba-plant. The prince did so, but at once spat it out, because of its intense bitterness. “If such bitterness should reside in the baby-tree, how will it be when it grows up?” asked the Bodhisatta, and thereupon drew a moral with regard to the prince’s own conduct. The prince benefited by the lesson, and thenceforth changed his nature.
The story was told in reference to a Licchavi-Kumāra called Duṭṭha. J.i.504-8.