The Epic of Gilgamesh, very similar to Noah’s Ark story, is the oldest known story. The story originated in Sumerian times around 2000 BC. Gilgamesh was the ruler of Uruk.
Translation of the story are available here.
It is interesting that Hindu mythology has a similar story of a great flood. One can find similar stories all over the ancient world.

Bridging the Myth and Science of the Flood
Flood Legends from Around the World
Flood Legends