CC Ādi 7.12

ithe bhakta-bhāva dhare caitanya gosāñi
‘bhakta-svarūpa’ tāṅra nityānanda-bhāi
Word for word: 
ithe — for this reason; bhakta-bhāva — the ecstasy of a devotee; dhare — accepts; caitanya — Lord Caitanya Mahāprabhu; gosāñi — the transcendental teacher; bhakta-svarūpa — exactly like a pure devotee; tāṅra — His; nityānanda — Lord Nityānanda; bhāi — brother.
Translation: 
For this reason Śrī Caitanya Mahāprabhu, the supreme teacher, accepts the form of a devotee and accepts Lord Nityānanda as His elder brother.