After millions of years, at the time of cosmic annihilation, when everything, manifested and unmanifested, is annihilated by the force of time, the five gross elements enter into the subtle conception, and the manifested categories enter into the unmanifested substance. At that time, You alone remain, and You are known as Ananta Śeṣa-nāga.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
नष्टे लोके द्विपरार्धावसाने महाभूतेष्वादिभूतं गतेषु । व्यक्तेऽव्यक्तं कालवेगेन याते भवानेकः शिष्यते शेषसंज्ञः
naṣṭe loke dviparārdhāvasāne mahābhūteṣvādibhūtaṃ gateṣu | vyakte'vyaktaṃ kālavegena yāte bhavānekaḥ śiṣyate śeṣasaṃjñaḥ
SB 10.3.25