CC Ādi 5.100-101

ananta-śayyāte tāṅhā karila śayana
sahasra mastaka tāṅra sahasra vadana
sahasra-caraṇa-hasta, sahasra-nayana
sarva-avatāra-bīja, jagat-kāraṇa
Synonyms: 
ananta-śayyāte — on Lord Ananta as a bed; tāṅhā — there; karila śayana — lay down; sahasra — thousands; mastaka — heads; tāṅra — His; sahasra vadana — thousands of faces; sahasra — thousands; caraṇa — legs; hasta — hands; sahasra-nayana — thousands of eyes; sarva-avatāra-bīja — the seed of all incarnations; jagat-kāraṇa — the cause of the material world.
Translation: 
He lay there with Ananta as His bed. Lord Ananta is a divine serpent having thousands of heads, thousands of faces, thousands of eyes and thousands of hands and feet. He is the seed of all incarnations and is the cause of the material world.
Purport: 

In the reservoir of water first created by the perspiration of Garbhodakaśāyī Viṣṇu, the Lord lies on the Śeṣa plenary expansion of Viṣṇu, who is described in Śrīmad-Bhāgavatam and in the four Vedas as follows:

sahasra-śīrṣā puruṣaḥsahasrākṣaḥ sahasra-pāt
sa bhūmiṁ viśvato vṛtvā-
tyatiṣṭhad daśāṅgulam

The Viṣṇu form called Ananta-śayana has thousands of hands and legs and thousands of eyes, and He is the active generator of all the incarnations within the material world.