From the Satapatha Brahmana
In the morning they brought Manu water for ablutions, iust as they bring it for washing the hands. As he was washing, a fish came into his hands and said, "Care for me and I will save you." From what will you save me? "A flood will carry away all these creatures; I will save you from it." "How should you be cared for?" "As long as we are tiny," said the fish, "our destruction is great, for fish swallows fish. Care for me at first in a pot, and when I outgrow it, dig a trench and care for me in it. And when I outgrow that, then take me down to the ocean, for then I will be beyond destruction."
The fish grew steadily into a jhasa, for that grows largest. It said, "In a certain year, the flood will come. Then you will build a ship and come to me, and when the flood has risen you will enter the ship and I will save you from the flood." Manu cared for it in this way and carried it down to the ocean. And in the very year which the fish had indicated, he bat a ship and came to him, and when the flood had risen he entered the ship. The fish swam up to him, and he fastened the rope of the ship to the horn of the fish, and with it he sailed through to the northern mountain. "I have saved you," said the fish. "Fasten the ship to a tree, but do not let the water cut you off when you are on the mountain; as the water subsides, keep following it down." And he kept following it down, in this way, and so that slope of the northern mountain is known as Manu's Descent. The flood swept away all other creatures, and Manu alone remained here.
Translation by Wendy Doniger O'Flaherty (Hindu Myths [New York: Penguin, 1975.])