The foremost manifestation of Kṛṣṇa is Saṅkarṣaṇa, who is known as Ananta. He is the origin of all incarnations within this material world. Previous to the appearance of Lord Kṛṣṇa, this original Saṅkarṣaṇa will appear as Baladeva, just to please the Supreme Lord Kṛṣṇa in His transcendental pastimes.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
वासुदेवकलानन्तः सहस्रवदनः स्वराट् । अग्रतो भविता देवो हरेः प्रियचिकीर्षया
vāsudevakalānantaḥ sahasravadanaḥ svarāṭ | agrato bhavitā devo hareḥ priyacikīrṣayā
SB 10.1.24