[37] Then the Blessed One, wandering by stages on tour, arrived at Uruvelā. Now at that time three matted-hair ascetics (jaṭila) were dwelling at Uruvelā: Uruvelakassapa, Nadīkassapa, and Gayākassapa. Among them, the jaṭila Uruvelakassapa was the leader, the guide, the foremost, the chief, the head of five hundred jaṭilas. The jaṭila Nadīkassapa was the leader, the guide, the foremost, the chief, the head of three hundred jaṭilas. The jaṭila Gayākassapa was the leader, the guide, the foremost, the chief, the head of two hundred jaṭilas.
Then the Blessed One approached the hermitage of the jaṭila Uruvelakassapa, and having approached, said this to the jaṭila Uruvelakassapa: "If it is not burdensome to you, Kassapa, we would dwell one night in the fire chamber."
"It is not burdensome to me, great ascetic; but there is a fierce nāga king there, possessed of psychic power, a venomous serpent of dreadful venom. May it not harm you."
A second time the Blessed One said this to the jaṭila Uruvelakassapa: "If it is not burdensome to you, Kassapa, we would dwell one night in the fire chamber."
"It is not burdensome to me, great ascetic; but there is a fierce nāga king there, possessed of psychic power, a venomous serpent of dreadful venom. May it not harm you."
A third time the Blessed One said this to the jaṭila Uruvelakassapa: "If it is not burdensome to you, Kassapa, we would dwell one night in the fire chamber."
"It is not burdensome to me, great ascetic; but there is a fierce nāga king there, possessed of psychic power, a venomous serpent of dreadful venom. May it not harm you."
"Perhaps it will not harm me. Come, Kassapa, grant me the fire chamber."
"Dwell there at your ease, great ascetic."
Then the Blessed One, having entered the fire chamber and spread a grass mat, sat down, having folded his legs crosswise, set his body erect, and established mindfulness before him.
English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.