Revākhaṇḍa ascribed to the Vāyupurāṇa (RKV) 231.13

231.13

nāgeśvarāśca saptaiva revātīradvaye 'pi tu /
saptaiva vahnivihitānyathāpyāvartasaptakam //

and on both banks of the Revā there are precisely seven Nāgeśvaras; / precisely seven were established by Vahni, and likewise a group of seven Āvartas;