Shatapatha-Brahmana, I, 8, 1-6.
1. In the morning they brought to Manu water for washing, just as now they continue to bring water for washing the hands. When Manu was washing himself, a fish came into his hands.
2. The fish spoke to him: “Protect me and I will save you!” “From what are you going to save me?” Manu asked. “A flood will carry away all everything: from that flood I will save you!” “How am I to protect you?” said Manu.
3. The fish said, “As long as we are small, there is great destruction for us: fish eat fish. You will first keep me in a jar. When I outgrow that, you will dig a pit and keep me in it. When I outgrow that, you will take me down to the sea, for then I shall be beyond destruction.”
4. It soon became a ghasha (a great fish), one that grows largest (of all fish). The great fish said, “In such and such a year that flood will come. You will then attend to me and by my advice prepare a ship and when the flood has risen you will enter into the ship, and I will save you from it.”
5. After Manu had reared the fish this way, he took it down to the sea. And in the same year which the fish had indicated to him, he followed the advice of the fish and built a ship. And when the flood had risen, he entered into the ship. The fish then swam up to him, and to its horn he tied the rope of the ship, and by that means he passed swiftly up to the northern mountains.
6. The fish then said, “I have saved you. Fasten the ship to a tree; but do not let the water cut you off while you are on the mountain. As the water subsides, you may gradually descend!” Accordingly Manu gradually descended and since then the slope of the northern mountain is called ‘Manu’s descent.’ The flood then swept away all creatures and Manu alone remained here.
Matsyavatara (The Fish Incarnation) [From the Bhagavata VIII]
There was an intermediate deluge. Brahma slept for a while and the demon Hayagriva stole the Vedas. Lord Vishnu noticed this and took the form of Fish. In the Dravida country, there was a pious King, Satyavrata by name. As he was making an offering of water in the Kritamala river, the Lord appeared as a tiny Fish in the water of his palm. The Fish began to grow, and wondering at this, the King went on transferring it from one container to another. The Fish, which had finally to be deposited in the sea, told him: “On the seventh day from now, all the worlds will become completely flooded; on the flood waters, a boat will come to you; embark in it with manifold herbs and seeds and surrounded by the seven great sages and every class of living beings; a strong gale will rock the boat, but tie it to my snout with the great serpent, and as you ask me questions, I shall expound to you then the glory of Myself, the Supreme Brahman.”
Accordingly the sea swelled as huge rain-clouds poured down incessantly, rolled on and engulfed the world; the boat appeared, and also the great Fish; to its single snout, Satyavrata tied the ark. Dragging the ark over the waters, the Lord as a Fish imparted to Satyavrata the teachings about Truth which were collected in the Purana known as the Matsya (Fish). After the waters of the deluge had subsided, the Lord slew the demon Hayagriva and restored the Vedas to Brahma, who had awoke from his slumber.