SB 1.3 - Kūrma, Dhanvantari and Mohinī
(11) The Lord took the form of a tortoise (Kūrma), whose shell served as a pivot for the Mandarācala Hill, which was used as a churning rod by the demigods and demons. Once both the atheists and the theists were engaged in producing nectar from the sea so that all of them could become immortal by drinking it. This was when the Lord appeared as a tortoise. The Padma Purāṇa says that the Kūrma who held up the Mandara mountain also picked up the earth from the lower regions on the request of the devatās. Viṣṇu-dharmottara says that the Kūrma who held up the earth appeared at the beginning of the day of Brahmā and then appeared to hold up the mountain during the Cākṣuṣa era.
(12) The Lord appeared as Dhanvantari to bring a pot of nectar. He appeared once during Cākṣuṣa era holding nectar from the churning of ocean in a pot, and started the Āyurveda, and appeared again the Vaivasvata era as the son of Kāśīrāja with the name Dhanvā.
(13) The Lord allured the atheists by the charming beauty of a woman (Mohinī) and gave nectar to the demigods to drink. He appeared twice – once for bewildering the demons and once for pleasing Śiva.