CC Madhya 19.172

lakṣaṇaṁ bhakti-yogasya
nirguṇasya hy udāhṛtam
ahaituky avyavahitā
yā bhaktiḥ puruṣottame
Word for word: 
lakṣaṇam — the symptom; bhakti-yogasya — of devotional service; nirguṇasya — beyond the three modes of nature; hi — certainly; udāhṛtam — is cited; ahaitukī — causeless; avyavahitā — uninterrupted; yā — which; bhaktiḥ — devotional service; puruṣa-uttame — to the Supreme Personality of Godhead.
Translation: 
“ ‘These are the characteristics of transcendental loving service to Puruṣottama, the Supreme Personality of Godhead: it is causeless, and it cannot be obstructed in any way.