Bhagavatapurana 5.26.10

5.26.10

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ye tv iha yathaivāmunā vihiṃsitā jantavaḥ paratra yama-yātanām upagataṃ ta eva ruravo bhūtvā tathā tam eva vihiṃsanti tasmād rauravam ity āhū rurur iti sarpād ati-krūra-sattvasyāpadeśaḥ //

11. The very beings who are here by any one subjected to torments and afflictions in any particular manner become Rurus and torment him in the same manner when he is down there to suffer Yama's punishment. Hence they call it Raurava. (Ruru is the name of a creature (worm) very much more cruel than the serpent).