Srimad Bhagavad-Gita :
Chapter-4. ( Jnana-karma-sanyasa-yogam )
Slokam-5. ( Lord Krishna replied : Many births of mine and also of your's passed, O Arjuna I am knowledgable all of them, but you are not. )
Sri-Bhagavan uvaca :
Bahuni me vyatitani janmani tava carjuna,
tanyaham veda sarvanni na tvam vettha parantapa.
Sri-Bhagavan uvaca = Lord replied (to Arjuna's question);
me tava ca = to Me and you;
bahuni janmani vyatitani = many births passed;
tani sarvanni aham veda = all those (births), I, know;
tvam na vettha parantapa = o arjuna you do not know.
Lord Krishna replying informs Arjuna that He instructed Visvavan in a different form then the one Arjuna is beholding at that moment and that by His supreme potency, immutable and imperishable He remembers it as well as every appearance that He ever manifested.
Lord Krishna understanding Arjuna's intention in posing this question explains that many births have passed meaning that they have both experienced many bodily manifestations. Lord Krishna is aware of them all as well as all other living beings because His knowledge is complete and omniscient but Arjuna's knowledge is veiled as well as all other living entities.
Lord Krishna's declaration of: many births have past for you and for Me is a definite affirmation of the reality of reincarnation for Arjuna and confirmation of the avatars or incarnations of the Supreme Lord Krishna. What He is confirming is that His present incarnation is as real as Arjuna's present life reincarnation and that the ones from the past are real as well, subtly implying that there is a fundamental difference between them. The nature of the avatars, how they manifest and their purpose for incarnating is given next slokam.
To be continued ...