A faithful summary of the paurāṇic story (not the verbatim text).
Satyanārāyaṇa (Viṣṇu) · Traditional devotional text (summary) · Source: Skanda Purāṇa, Revā-khaṇḍa — summarised (Summary of a public-domain paurāṇic text)
The Satyanārāyaṇa Vrata is described in the Skanda Purāṇa, where the sage Nārada asks Viṣṇu how the suffering of the world may be eased. Viṣṇu prescribes the worship of Satyanārāyaṇa — the Lord as Truth — on a Pūrṇimā or Saṅkrānti, with devotion and the reading of his kathā.
The kathā is told through a chain of devotees — a poor brāhmaṇa, a woodcutter, the merchant Sādhu Vaṇik and his daughter Kalāvatī, and King Aṅgadhvaja. Each prospers when they keep faith with the vow and offer the sacred prasāda, and each meets misfortune when they neglect or scorn it — until, remembering the Lord sincerely, they are restored.
The moral the Purāṇa draws is simple: steadfast truth and humble devotion, not wealth or status, secure the Lord's grace. The vrata is kept to this day at housewarmings, weddings and thanksgivings.
Offered for worship and study, not as prediction. Non-commercial.