Certainly, therefore, since time immemorial, all transcendentalists have been rendering devotional service to Lord Kṛṣṇa, the Personality of Godhead, with great delight, because such devotional service is enlivening to the self.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
अतो वै कवयो नित्यं भक्तिं परमया मुदा । वासुदेवे भगवति कुर्वन्त्यात्मप्रसादनीम्
ato vai kavayo nityaṃ bhaktiṃ paramayā mudā | vāsudeve bhagavati kurvantyātmaprasādanīm
SB 1.2.22