CC Ādi 4.118
mathurāyāṁ madhurākṣi rādhikā ca
abhaviṣyad iyaṁ vṛthā visṛṣṭir
makarāṅkas tu viśeṣatas tadātra
hariḥ — Lord Kṛṣṇa; eṣaḥ — this; na — not; cet — if; avātariṣyat — would have descended; mathurāyām — in Mathurā; madhura-akṣi — O lovely-eyed one (Paurṇamāsī); rādhikā — Śrīmatī Rādhikā; ca — and; abhaviṣyat — would have been; iyam — this; vṛthā — useless; visṛṣṭiḥ — the whole creation; makara-aṅkaḥ — the demigod of love, Cupid; tu — then; viśeṣataḥ — above all; tadā — then; atra — in this.
“O Paurṇamāsī, if Lord Hari had not descended in Mathurā with Śrīmatī Rādhārāṇī, this entire creation — and especially Cupid, the demigod of love — would have been useless.”
This verse is spoken by Śrī Vṛndā-devī in the Vidagdha-mādhava (7.3) of Śrīla Rūpa Gosvāmī.