CC Ādi 5.120

sei ta’ ‘ananta’ ‘śeṣa’ — bhakta-avatāra
īśvarera sevā vinā nāhi jāne āra
Word for word: 
sei ta’ — that; ananta — Lord Ananta; śeṣa — the incarnation Śeṣa; bhakta-avatāra — incarnation of a devotee; īśvarera sevā — the service of the Lord; vinā — without; nāhi — not; jāne — knows; āra — anything else.
Translation: 
That Ananta Śeṣa is the devotee incarnation of Godhead. He knows nothing but service to Lord Kṛṣṇa.
Purport: 

Śrīla Jīva Gosvāmī, in his Kṛṣṇa-sandarbha, has described Śeṣa Nāga as follows: “Śrī Anantadeva has thousands of faces and is fully independent. Always ready to serve the Supreme Personality of Godhead, He waits upon Him constantly. Saṅkarṣaṇa is the first expansion of Vāsudeva, and because He appears by His own will, He is called svarāṭ, fully independent. He is therefore infinite and transcendental to all limits of time and space. He Himself appears as the thousand-headed Śeṣa.” In the Skanda Purāṇa, in the Ayodhyā-māhātmya chapter, the demigod Indra requested Lord Śeṣa, who was standing before him as Lakṣmaṇa, “Please go to Your eternal abode, Viṣṇuloka, where Your expansion Śeṣa, with His serpentine hoods, is also present.” After thus dispatching Lakṣmaṇa to the regions of Pātāla, Lord Indra returned to his abode. This quotation indicates that the Saṅkarṣaṇa of the quadruple form descends with Lord Rāma as Lakṣmaṇa. When Lord Rāma disappears, Śeṣa again separates Himself from the personality of Lakṣmaṇa. Śeṣa then returns to His own abode in the Pātāla regions, and Lakṣmaṇa returns to His abode in Vaikuṇṭha.

The Laghu-bhāgavatāmṛta gives the following description: “The Saṅkarṣaṇa of the second group of quadruple forms appears as Rāma, taking with Him Śeṣa, who bears the global spheres. There are two features of Śeṣa. One is the bearer of the globes, and the other is the bedstead servitor. The Śeṣa who bears the globes is a potent incarnation of Saṅkarṣaṇa, and therefore He is sometimes also called Saṅkarṣaṇa. The bedstead feature of Śeṣa always presents himself as an eternal servitor of the Lord.”