CC Madhya 19.170
sevanaṁ bhaktir ucyate
sarva-upādhi-vinirmuktam — free from all kinds of material designations, or free from all desires except the desire to render service to the Supreme Personality of Godhead; tat-paratvena — by the sole purpose of serving the Supreme Personality of Godhead; nirmalam — uncontaminated by the effects of speculative philosophical research or fruitive activity; hṛṣīkeṇa — by purified senses freed from all designations; hṛṣīka-īśa — of the master of the senses; sevanam — the service to satisfy the senses; bhaktiḥ — devotional service; ucyate — is called.
“ ‘Bhakti, or devotional service, means engaging all our senses in the service of the Lord, the Supreme Personality of Godhead, the master of all the senses. When the spirit soul renders service unto the Supreme, there are two side effects. One is freed from all material designations, and one’s senses are purified simply by being employed in the service of the Lord.’
This verse quoted from the Nārada-pañcarātra is found in the Bhakti-rasāmṛta-sindhu (1.1.12).