CC Ādi 5.152

kṛṣṇa-avatāre jyeṣṭha hailā sevāra kāraṇa
kṛṣṇake karāila nānā sukha āsvādana
Word for word: 
kṛṣṇa-avatāre — in the incarnation of Lord Kṛṣṇa; jyeṣṭha hailā — He became the elder brother; sevāra kāraṇa — for the purpose of service; kṛṣṇake — to Kṛṣṇa; karāila — made; nānā — various; sukha — happinesses; āsvādana — tasting.
Translation: 
When Lord Kṛṣṇa appeared, He [Balarāma] became His elder brother to serve Him to His heart’s content and make Him enjoy all sorts of happiness.