The legend of Āruṇi (Aruni), disciple of Āpoddhaumya(Apoddhaumya) is one of the famous legends of gurubhakti (loyalty and respect towards the guru) mentioned in Mahābhārata (Mahabharata). This is a popular legend since the ancient times. This has been narrated in Ādiparva(Adiparva) of Mahābhārata.
In ancient times, there was a seer-sage called Āpoddhaumya. Āruṇi of Pāñcāla(Panchala) was one of the major three disciples of Āpoddhaumya. Once the sage ordered Āruṇi to put a barrier to a broken ridge. Immediately Āruṇi tried to do accordingly, but whenever he pried to set up a barrier against the broken ridge, it broke yup again, and water was oozing out through it. Feeling distressed, Āruni started thinking what to do. After some time, he got an idea. he himself lied down against the broken portion of the ridge. and prevented the water from leaking out. He remained in this posture for a long time. When Sage Āpoddhaumya asked his other disciples, Veda and Upamanyu, about Āruṇi, they said that the guru himself sent him to set a barrier against the broken ridge. Then Āpoddhaumya, along with his other two disciples, went to the place, and started called Āruṇi by his name. hearing the guru’s voice Āruṇi rose from the ridge and came in front of him. He informed the guru that finding no other way of preventing the water from leaking out, he himself lied down there. At the guru’s call, he had just risen from that very place.  Āpoddhaumya was very please at this loyalty of his disciple, and said that Āruni would be famous as Uddālaka (Uddalaka), because he had come up, rising from the broken ridge. He also blessed him, saying that he would be entitled to the knowledge of all the Vedas, for his diligent performance according to the guru’s order.

[See Uddālaka (Uddalaka)]

In Chāndogya Upaniṣad (Chandogya Upanishad), too, we have the mention of one Āruṇi. He gave Śvetaketu (Swetaketu) many advices about Suṣupti(Sushupti) ans Brahmacarya.

Āruṇi is also referred to as one of the Siddhapuruṣa (Siddhapurusha, learned sages who have attained siddhi) mentioned in Bhāgavatapurāṇa (Bhagavatapurana).