Mbh 1.216.3a

“Listen now, O Janamejaya, to a wonderful feat which that foremost of the sons of Pandu, of high soul, did, while living there. When that son of Kunti, O Bharata, and the Brahmanas who had followed him, took up their residence in that region, the latter performed innumerable Agnihotras (sacrificial rites by igniting the sacred fire). And, O king, in consequence of those learned vow-observing, and illustrious Brahmanas, who never deviated from the right path, daily establishing and igniting with mantras on the banks of that sacred stream, after the performance of their ablutions, fires for their sacrifices, and pouring libations of clarified butter into the same, and worshipping those fires with offerings of flowers, that region itself where the Ganges entered the plains became exceedingly beautiful. One day that bull amongst the Pandavas, while residing in that region in the midst of those Brahmanas, descended (as usual) into the Ganges to perform his ablutions. After his ablutions had been over, and after he had offered oblations of water unto his deceased ancestors, he was about to get up from the stream to perform his sacrificial rites before the fire, when the mighty-armed hero, O king, was dragged into the bottom of the water by Ulupi, the daughter of the king of the Nagas, urged by the god of desire. And it so happened that the son of Pandu was carried into the beautiful mansion of Kauravya, the king of the Nagas. Arjuna saw there a sacrificial fire ignited for himself. Beholding that fire, Dhananjaya, the son of Kunti performed his sacrificial rites with devotion. And Agni was much gratified with Arjuna for the fearlessness with which that hero had poured libations into his manifest form. After he had thus performed his rites before the fire, the son of Kunti, beholding the daughter of the king of the Nagas, addressed her smilingly and said, ‘O handsome girl, what an act of rashness hast thou done. O timid one!