Vishnu Puran 15 (with English subtitle)

By the suggestion of God Vishnu, Indra god convinces demon teacher Shukracharya that nectar is superior than sanjeevini chant and makes them ready for chumming of ocean. God Vishnu arranges Vasuki snake and Mandar mount for chumming of ocean. Narad convinces demons to hold Vasuki's head side by praising them. When Mandar sinks, God Vishnu takes incarnation of Turtle to hold Mandar on His back.