CC Ādi 2.114

keho kahe, kṛṣṇa kṣīroda-śāyī avatāra
asambhava nahe, satya vacana sabāra
Word for word: 
keho — someone; kahe — says; kṛṣṇa — Lord Kṛṣṇa; kṣīroda-śāyī — Kṣīrodakaśāyī Viṣṇu; avatāra — incarnation; asambhava — impossible; nahe — is not; satya — true; vacana — speeches; sabāra — of all.
Translation: 
Some say that Kṛṣṇa is the incarnation of Kṣīrodakaśāyī Viṣṇu. None of these statements is impossible; each is as correct as the others.
Purport: 

The Laghu-bhāgavatāmṛta (5.383) states:

ata evā purāṇādaukecin nara-sakhātmatām
mahendrānujatāṁ kecit
kecit kṣīrābdhi-śāyitām

sahasra-śīrṣatāṁ kecitkecid vaikuṇṭha-nāthatām
brūyuḥ kṛṣṇasya munayas
tat-tad-vṛtty-anugāminaḥ

“According to the intimate relationships between Śrī Kṛṣṇa, the primeval Lord, and His devotees, the Purāṇas describe Him by various names. Sometimes He is called Nārāyaṇa; sometimes Upendra (Vāmana), the younger brother of Indra, King of heaven; and sometimes Kṣīrodakaśāyī Viṣṇu. Sometimes He is called the thousand-hooded Śeṣa Nāga, and sometimes the Lord of Vaikuṇṭha.”