Matsya Purana — Catalogue of the Eighteen Puranas
ब्राह्मं त्रिदशसाहस्रं पुराणं परिकीर्त्यते लिखित्वा तच्च यो दद्याज् जलधेनुसमन्वितम् वैशाखपूर्णिमायां च ब्रह्मलोके महीयते //
brāhmaṃ tridaśasāhasraṃ purāṇaṃ parikīrtyate likhitvā tacca yo dadyāj jaladhenusamanvitam vaiśākhapūrṇimāyāṃ ca brahmaloke mahīyate //
The Brahma Purāṇa is proclaimed to be thirty thousand verses in extent. Whoever has it written out and then donates that manuscript—together with a jaladhenu (“water-cow,” a ritual gift for the merit of providing water)—on the full-moon day of Vaiśākha is honored in Brahmaloka, the world of Brahmā.
This verse does not discuss Pralaya; it focuses on dāna-dharma—specifically the merit gained by donating a written Purāṇa along with a jaladhenu on Vaiśākha Pūrṇimā.
It frames a householder’s (and by extension a king’s) dharmic duty of supporting sacred learning: commissioning a manuscript and gifting it, with an auxiliary ritual gift (jaladhenu), as a high-merit act that yields exalted posthumous status (honor in Brahmaloka).
The ritual significance is the prescribed timing and pairing of gifts: donating a Purāṇa manuscript specifically on Vaiśākha Pūrṇimā, accompanied by jaladhenu—an established dāna symbolizing the provision of water and sustaining merit.