CC Ādi 5.107

hena nārāyaṇa, — yāṅra aṁśera aṁśa
sei prabhu nityānanda — sarva-avataṁsa
Word for word: 
hena — such; nārāyaṇa — Lord Nārāyaṇa; yāṅra — of whom; aṁśera — of the plenary part; aṁśa — a part; sei — that; prabhu — the Lord; nityānanda — Nityānanda; sarva-avataṁsa — the source of all incarnations.
Translation: 
That Lord Nārāyaṇa is a part of a plenary part of Lord Nityānanda Balarāma, who is the source of all incarnations.