Matsya Purana — Procedure for Going to Prayaga and the Greatness of the Ganga
कम्बलाश्वतरौ नागौ विपुले यमुनातटे तत्र स्नात्वा च पीत्वा च सर्वपापैः प्रमुच्यते //
kambalāśvatarau nāgau vipule yamunātaṭe tatra snātvā ca pītvā ca sarvapāpaiḥ pramucyate //
On the broad bank of the Yamunā are the two Nāgas, Kambala and Aśvatara; by bathing there and drinking the water, one is released from all sins.
This verse does not discuss pralaya; it teaches tīrtha-mahātmya—how bathing and drinking at a specific Yamunā sacred spot grants purification from sin.
It supports the dharmic duty of purification through approved rites: householders (and rulers) are encouraged to perform tīrtha-snānā as part of expiation and maintenance of personal and social purity.
The ritual point is primary: snāna (sacred bathing) and pāna (drinking the tīrtha water) at the Yamunā bank associated with Nāga-deities is said to remove sins; no vastu/temple-building rule is stated.