CC Madhya 23.8
bhaktir ity ucyate bhīṣma-
ananya-mamatā — having a sense of relationships with no others; viṣṇau — in Lord Viṣṇu, or Kṛṣṇa; mamatā — the sense of ownership; prema-saṅgatā — endowed only with love; bhaktiḥ — devotional service; iti — thus; ucyate — is said; bhīṣma — by Bhīṣma; prahlāda — by Prahlāda Mahārāja; uddhava — by Uddhava; nāradaiḥ — and by Nārada.
“ ‘When one develops an unflinching sense of ownership or possessiveness in relation to Lord Viṣṇu, or, in other words, when one thinks Viṣṇu and no one else to be the only object of love, such an awakening is called bhakti [devotion] by exalted persons like Bhīṣma, Prahlāda, Uddhava and Nārada.’
This verse, quoted from the Nārada-pañcarātra, is found in the Bhakti-rasāmṛta-sindhu (1.4.2).