CC Ādi 5.8

śrī-balarāma gosāñi mūla-saṅkarṣaṇa
pañca-rūpa dhari’ karena kṛṣṇera sevana
Word for word: 
śrī-balarāma — Balarāma; gosāñi — the Lord; mūla-saṅkarṣaṇa — the original Saṅkarṣaṇa; pañca-rūpa dhari’ — accepting five bodies; karena — does; kṛṣṇera — of Lord Kṛṣṇa; sevana — service.
Translation: 
Lord Balarāma is the original Saṅkarṣaṇa. He assumes five other forms to serve Lord Kṛṣṇa.