CC Madhya 25.134
kṛṣṇas tu bhagavān svayam
mṛḍayanti yuge yuge
ete — these; ca — and; aṁśa — plenary portions; kalāḥ — parts of plenary portions; puṁsaḥ — of the puruṣa-avatāras; kṛṣṇaḥ — Lord Kṛṣṇa; tu — but; bhagavān — the Supreme Personality of Godhead; svayam — Himself; indra-ari — the enemies of Lord Indra; vyākulam — full of; lokam — the world; mṛḍayanti — make happy; yuge yuge — at the right time in each age.
“ ‘All these incarnations of Godhead are either plenary portions or parts of the plenary portions of the puruṣa-avatāras. But Kṛṣṇa is the Supreme Personality of Godhead Himself. In every age He protects the world through His different features when the world is disturbed by the enemies of Indra.’
This is a quotation from Śrīmad-Bhāgavatam (1.3.28). For an explanation, see Ādi-līlā, chapter two, text 67.