CC Madhya 23.47
kṛṣṇa-bhakti rasa-rūpe pāya pariṇāme
prema-ādika — love of Godhead, beginning with śānta, dāsya and so on; sthāyi-bhāva — the permanent ecstasies; sāmagrī-milane — by mixing with other ingredients; kṛṣṇa-bhakti — devotional service to Lord Kṛṣṇa; rasa-rūpe — composed of transcendental mellows; pāya — becomes; pariṇāme — by transformation.
“When the permanent ecstasies [neutrality, servitorship and so on] are mixed with other ingredients, devotional service in love of Godhead is transformed and becomes composed of transcendental mellows.
In the Bhakti-rasāmṛta-sindhu (2.1.4-5), the following definition is given:
sāmagrī-paripoṣeṇa paramā rasa-rūpatā
svādyatvaṁ hṛdi bhaktānām ānītā śravaṇādibhiḥ
eṣā kṛṣṇa-ratiḥ sthāyī bhāvo bhakti-raso bhavet
“Love for Kṛṣṇa, Keśava, as previously described, reaches the supreme state of being composed of mellows when its ingredients are fulfilled. By means of vibhāva, anubhāva, sāttvika and vyabhicārī, hearing and chanting are activated, and the devotee is able to taste love for Kṛṣṇa. Then attachment for Kṛṣṇa, or permanent ecstasy (sthāyi-bhāva), becomes the mellow of devotional service (bhakti-rasa).”