SB 10.3.48-49

tayā hṛta-pratyaya-sarva-vṛttiṣu
 dvāḥ-stheṣu paureṣv api śāyiteṣv atha
dvāraś ca sarvāḥ pihitā duratyayā
 bṛhat-kapāṭāyasa-kīla-śṛṅkhalaiḥ
tāḥ kṛṣṇa-vāhe vasudeva āgate
 svayaṁ vyavaryanta yathā tamo raveḥ
vavarṣa parjanya upāṁśu-garjitaḥ
 śeṣo ’nvagād vāri nivārayan phaṇaiḥ
Synonyms: 
tayā — by the influence of Yoga-māyā; hṛta-pratyaya — deprived of all sensation; sarva-vṛttiṣu — having all their senses; dvāḥ-stheṣu — all the doormen; paureṣu api — as well as other members of the house; śāyiteṣu — sleeping very deeply; atha — when Vasudeva tried to take his transcendental son out of the confinement; dvāraḥ ca — as well as the doors; sarvāḥ — all; pihitāḥ — constructed; duratyayā — very hard and firm; bṛhat-kapāṭa — and on great doors; āyasa-kīla-śṛṅkhalaiḥ — strongly constructed with iron pins and closed with iron chains; tāḥ — all of them; kṛṣṇa-vāhe — bearing Kṛṣṇa; vasudeve — when Vasudeva; āgate — appeared; svayam — automatically; vyavaryanta — opened wide; yathā — as; tamaḥ — darkness; raveḥ — on the appearance of the sun; vavarṣa — showered rain; parjanyaḥ — the clouds in the sky; upāṁśu-garjitaḥ — very mildly resounding and raining very slightly; śeṣaḥ — Ananta-nāga; anvagāt — followed; vāri — showers of rain; nivārayan — stopping; phaṇaiḥ — by spreading His hoods.
Translation: 
By the influence of Yoga-māyā, all the doorkeepers fell fast asleep, their senses unable to work, and the other inhabitants of the house also fell deeply asleep. When the sun rises, the darkness automatically disappears; similarly, when Vasudeva appeared, the closed doors, which were strongly pinned with iron and locked with iron chains, opened automatically. Since the clouds in the sky were mildly thundering and showering, Ananta-nāga, an expansion of the Supreme Personality of Godhead, followed Vasudeva, beginning from the door, with hoods expanded to protect Vasudeva and the transcendental child.
Purport: 

Śeṣa-nāga is an expansion of the Supreme Personality of Godhead whose business is to serve the Lord with all necessary paraphernalia. When Vasudeva was carrying the child, Śeṣa-nāga came to serve the Lord and protect Him from the mild showers of rain.