CC Ādi 17.12

tabe nityānanda-svarūpera āgamana
prabhuke miliyā pāila ṣaḍ-bhuja-darśana
Synonyms: 
tabe — thereafter; nityānanda-svarūpera — of the Personality of Godhead Nityānanda; āgamana — appearance; prabhuke — Lord Caitanya Mahāprabhu; miliyā — meeting; pāila — obtained; ṣaṭ-bhuja-darśana — a vision of the six-armed Śrī Caitanya Mahāprabhu.
Translation: 
After this function at the house of Śrīvāsa Ṭhākura, Nityānanda Prabhu appeared, and when He met with Lord Caitanya He got the opportunity to see Him in His six-armed form.
Purport: 

The form of Ṣaḍ-bhuja, the six-armed Lord Gaurasundara, is a representation of three incarnations. The form of Śrī Rāmacandra is symbolized by a bow in one hand and an arrow in another, the form of Lord Śrī Kṛṣṇa is symbolized by a stick and a flute like those generally held by a cowherd boy, and Lord Caitanya Mahāprabhu is symbolized by a sannyāsa-daṇḍa and a kamaṇḍalu, or waterpot.

Śrīla Nityānanda Prabhu was born in the village of Ekacakrā, in the district of Birbhum, as the son of Padmāvatī and Hāḍāi Paṇḍita. In His childhood He played like Balarāma. When He was growing up, a sannyāsī came to the house of Hāḍāi Paṇḍita and begged to have the paṇḍita’s son as his brahmacārī assistant. Hāḍāi Paṇḍita immediately agreed and delivered his son to him, although the separation was greatly shocking, so much so that Hāḍāi lost his life after the separation. Nityānanda Prabhu traveled on many pilgrimages with the sannyāsī. It is said that for many days He lived at Mathurā with him, and at that time He heard about Lord Caitanya Mahāprabhu’s pastimes in Navadvīpa. Therefore He came down to Bengal to see the Lord. When Lord Nityānanda came to Navadvīpa, He was a guest at the house of Nandana Ācārya. Understanding that Nityānanda Prabhu had arrived, Lord Caitanya sent His devotees to Him, and thus there was a meeting between Śrī Caitanya Mahāprabhu and Nityānanda Prabhu.