Srimad Bhagvata Mahapurana: Book 8: Chapter 24: Verses 32-48
The story of the descent of the divine Fish recounted
The glorious Lord said : "On the seventh day following this day, O subduer of foes, (all) these three worlds-Bhuloka (the terrestrial region), the Bhuvarloka (the aerial region) and heaven-will be submerged in the ocean rising for the dissolution of the universe. When the three worlds are actually going to be submerged in the deluge water, a mysterious and spacious boat, sent by Me, will approach you. Take (with you) in the meanwhile all the herbs and annual plants as well as seeds of all types (both great and small) and, surrounded by the seven seers, and accompanied by all (varieties of) animals, you shall board that commodious vessel and sail about undaunted in that undivided expanse of water devoid of light, guided by the effulgence of the Rsis alone. Attach that vessel-tossed about by a mighty gale-by means of the great serpent (Vasuki) to My horn as I shall be present by your side (at that time). Pulling you, boat and all, along with the (seven) seers, I shall roam about in the vast expanse of water as long as the night of Brahma (which is of the same duration as his day) lasts, O king!" And (at that time) you will (come to) realize My glory--designated by the name of Para Brahma (the transcendent Reality)-revealed by Me in your heart through (answer to) your questions, though (actually) imparted by My grace.
Having thus instructed the king (Satyavrata), Sri Hari disappeared; (while) the former awaited the time which Lord Visnu (the Controller of our senses) had already told him. Spreading (on the ground) blades of (the sacred) Kusa grass with their ends pointing towards the east, the royal sage sat down with his face turned towards the north-east, contemplating on the feet of Lord Sri Hari disguised as the divine Fish. Then was the ocean clearly seen rising high on account of huge clouds pouring down (heavy) rain and (consequently) overflowing its limits and inundating the earth on all sides. Revolving in his mind the command of the Lord, the king perceived the vessel arrived (by his side).Taking the herbs and plants (collected by him), he boarded it along with the great sages. The sages lovingly said to him, "Meditate on Lord Visnu (the Ruler of Brahma and Lord Siva as well), O (good) king! He will surely deliver us from this calamity and bring happiness (to us).
Thought of uninterruptedly by the king, the Lord then appeared in that vast expanse of water in the form of a golden (effulgent) whale, possessed of a horn and extending over an area of one lakh Yojanas (or eight lakh miles). Fastening the boat to the horn of the divine whale with the king of serpents for a rope, as already enjoined by Sri Hari, king Satyavrata, full of joy, hymned (in the following words) Lord Visnu (the Slayer of the demon Madhu).The king prayed : People whose true knowledge of their own self stands obscured by dateless ignorance and who are exhausted by the toils (and turmoils) of worldly existence, traceable to that nescience, realize You here (only) when they take shelter in You by Your unaccountable grace. Such as You are, may You act as our highest preceptor and prove to be the Bestower of final beatitude (on us). This ignorant Jiva (human soul), bound (as it is) by its own (previous) Karma, performs actions with great pains in the hope of (attaining) happiness and is able to get rid of that hope as well as of its wrong notion (in the shape of identification with body etc.), through Your worship (alone). May that Lord, who is our (true) guide, snap our knot (of ignorance) existing in the heart. Through His worship (alone) can an embodied soul shake off its impurity in the shape of ignorance and regain its essential (blissful) character, (even) as silver and gold shed their impurity and regain their native brilliance through contact with fire (alone). May that immortal Lord, who is (even) higher than a Guru, be our (real) preceptor.