CC Madhya 23.46
ye-rase bhakta ‘sukhī’, kṛṣṇa haya ‘vaśa’
“These five transcendental mellows exist permanently. The devotee may be attracted to one of these mellows, and thus he becomes happy. Kṛṣṇa also becomes inclined toward such a devotee and comes under his control.
In the Bhakti-rasāmṛta-sindhu (2.5.1), sthāyi-bhāva, permanent ecstasy, is thus described:
su-rājeva virājeta sa sthāyī bhāva ucyate
sthāyī bhāvo ’tra sa proktaḥ śrī-kṛṣṇa-viṣayā ratiḥ
“These moods (bhāvas) bring under control the favorable ecstasies (such as laughing) and unfavorable ecstasies (such as anger). When these moods continue to remain as kings, they are called sthāyi-bhāva, or permanent ecstasies. Continuous ecstatic love for Kṛṣṇa is called permanent ecstasy.”