Srimad-Bhagavatam: Canto 1 - Chapter 1 - Verse 23
एकोनविंशे विंशतिमे वृष्णिषु प्राप्य जन्मनी ।रामकृष्णाविति भुवो भगवानहरद्भरम् ॥ २३ ॥
ekonaviṁśe viṁśatimevṛṣṇiṣu prāpya janmanīrāma-kṛṣṇāv iti bhuvobhagavān aharad bharam
In the nineteenth and twentieth incarnations, the Lord advented Himself as Lord Balarāma and Lord Kṛṣṇa in the family of Vṛṣṇi [the Yadu dynasty], and by so doing He removed the burden of the world.
The specific mention of the word bhagavān in this text indicates that Balarāma and Kṛṣṇa are original forms of the Lord. This will be further explained later. Lord Kṛṣṇa is not an incarnation of the puruṣa, as we learned from the beginning of this chapter. He is directly the original Personality of Godhead, and Balarāma is the first plenary manifestation of the Lord. From Baladeva the first phalanx of plenary expansions, Vāsudeva, Saṅkarṣaṇa, Aniruddha and Pradyumna, expands. Lord Śrī Kṛṣṇa is Vāsudeva, and Baladeva is Saṅkarṣaṇa.