Matsya Purana — Bhīma-Dvādaśī
कल्याणिनी नाम पुरा बभूव या द्वादशी माघदिनेषु पूज्या सा पाण्डुपुत्रेण कृता भविष्यत्य् अनन्तपुण्यानघ भीमपूर्वा //
kalyāṇinī nāma purā babhūva yā dvādaśī māghadineṣu pūjyā sā pāṇḍuputreṇa kṛtā bhaviṣyaty anantapuṇyānagha bhīmapūrvā //
Formerly there was a sacred observance known as Kalyāṇinī, a Dvādaśī (the twelfth lunar day) to be worshipped during the days of Māgha. That very rite was performed by the son of Pāṇḍu—Bhīma; and, O sinless one, it will become a source of endless merit.
This verse does not discuss Pralaya; it focuses on vrata-dharma, specifically the worship of a Māgha Dvādaśī called Kalyāṇinī and its inexhaustible merit.
It presents a model of dharmic practice: observing calendrical vows (vratas) and performing worship on sacred tithis to accumulate puṇya—an ethic applicable to householders and rulers alike as part of righteous living.
The ritual significance is the sanctity of Māgha Dvādaśī worship (Kalyāṇinī). No Vāstu or temple-architecture rule is stated, but the verse anchors a specific tithi-based pūjā as a high-merit rite.