SB 5.3.18

tata āgnīdhrīye ’ṁśa-kalayāvatariṣyāmy ātma-tulyam anupalabhamānaḥ.
Synonyms: 
tataḥ — therefore; āgnīdhrīye — in the wife of Nābhi, the son of Āgnīdhra; aṁśa-kalayā — by an expansion of My personal form; avatariṣyāmi — I shall advent Myself; ātma-tulyam — My equal; anupalabhamānaḥ — not finding.
Translation: 
Since I cannot find anyone equal to Me, I shall personally expand Myself into a plenary portion and thus advent Myself in the womb of Merudevī, the wife of Mahārāja Nābhi, the son of Āgnīdhra.
Purport: 

This is an example of the omnipotence of the Supreme Personality of Godhead. Although He is one without a second, He expands Himself by svāṁśa, His personal expansion, and sometimes by vibhinnāṁśa, or His separated expansion. Lord Viṣṇu herein agrees to send His personal expansion as the son of Merudevī, the wife of Mahārāja Nābhi, who is the son of Āgnīdhra. The ṛtvijaḥ, the priests, knew that God is one, yet they prayed for the Supreme Lord to become the son of Mahārāja Nābhi to let the world know that the Absolute Truth, the Supreme Personality of Godhead, is one without a second. When He incarnates, He expands Himself in different potencies.