CC Ādi 5.154

sei aṁśa lañā jyeṣṭha-kaniṣṭhābhimāna
aṁśāṁśi-rūpe śāstre karaye vyākhyāna
Word for word: 
sei aṁśa lañā — taking that plenary portion; jyeṣṭha-kaniṣṭha-abhimāna — considering Themselves the elder or younger; aṁśa-aṁśi-rūpe — as the expansion and the original Supreme Personality of Godhead; śāstre — in the revealed scriptures; karaye — does; vyākhyāna — explanation.
Translation: 
Kṛṣṇa and Balarāma present Themselves as younger brother and elder brother, but in the scriptures They are described as the original Supreme Personality of Godhead and His expansion.