CC Madhya 22.83

kṛṣṇa-bhakti-janma-mūla haya ‘sādhu-saṅga’
kṛṣṇa-prema janme, teṅho punaḥ mukhya aṅga
Word for word: 
kṛṣṇa-bhakti — of devotional service to Kṛṣṇa; janma-mūla — the root cause; haya — is; sādhu-saṅga — association with advanced devotees; kṛṣṇa-prema — love of Kṛṣṇa; janme — awakens; teṅho — that same association with devotees; punaḥ — again; mukhya aṅga — the chief principle.
Translation: 
“The root cause of devotional service to Lord Kṛṣṇa is association with advanced devotees. Even when one’s dormant love for Kṛṣṇa awakens, association with devotees is still most essential.