Agni Purana, Part One: The Avataras

Agni Purana

They started to churn the Ocean of Milk by using Mount Mandara as the pivot. When it started to sink Vishnu intervened in his incarnation as the tortoise Kurma, and offered the back of his shell to support the mountain. The divine serpent Vasuki, with five heads, offered himself as the rope and curled himself around the pivot. The Ocean of Milk was churned for another 1000 years before producing the much-desired elixir and other treasures including the goddess Lakshmi and the Apsaras
When the devas and asuras fought once again, Vishnu intervened, and with his help the devas won possession of the elixir.