CC Madhya 23.4

kṛṣṇe rati gāḍha haile ‘prema’-abhidhāna
kṛṣṇa-bhakti-rasera ei ‘sthāyi-bhāva’-nāma
Word for word: 
kṛṣṇe — unto Lord Kṛṣṇa; rati — affection; gāḍha — deep; haile — when it becomes; prema-abhidhāna — called love of God; kṛṣṇa-bhakti-rasera — of the mellows of devotional service to Kṛṣṇa; ei — this; sthāyi-bhāva-nāma — called sthāyi-bhāva.
Translation: 
“When affection for Kṛṣṇa becomes deeper, one attains love of Godhead in devotional service. Such a position is called sthāyi-bhāva, permanent enjoyment of the mellows of devotional service to Kṛṣṇa.