CC Ādi 5.48

turīya, viśuddha-sattva, ‘saṅkarṣaṇa’ nāma
tiṅho yāṅra aṁśa, sei nityānanda-rāma
Word for word: 
turīya — transcendental; viśuddha-sattva — pure existence; saṅkarṣaṇa nāma — named Saṅkarṣaṇa; tiṅho yāṅra aṁśa — of whom that Saṅkarṣaṇa is also a partial expansion; sei nityānanda-rāma — that person is known as Balarāma or Nityānanda.
Translation: 
That Saṅkarṣaṇa, who is transcendental pure goodness, is a partial expansion of Nityānanda Balarāma.