CC Madhya 17.136
na bhaved grāhyam indriyaiḥ
sevonmukhe hi jihvādau
svayam eva sphuraty adaḥ
ataḥ — therefore (because Kṛṣṇa’s name, form and qualities are all on the absolute platform); śrī-kṛṣṇa-nāma-ādi — Lord Kṛṣṇa’s name, form, qualities, pastimes and so on; na — not; bhavet — can be; grāhyam — perceived; indriyaiḥ — by the blunt material senses; sevā-unmukhe — to one engaged in His service; hi — certainly; jihvā-ādau — beginning with the tongue; svayam — personally; eva — certainly; sphurati — become manifest; adaḥ — those (Kṛṣṇa’s name, form, qualities and so on).
“ ‘Therefore material senses cannot appreciate Kṛṣṇa’s holy name, form, qualities and pastimes. When a conditioned soul is awakened to Kṛṣṇa consciousness and renders service by using his tongue to chant the Lord’s holy name and taste the remnants of the Lord’s food, the tongue is purified, and one gradually comes to understand who Kṛṣṇa really is.’
This verse is recorded in the Bhakti-rasāmṛta-sindhu (1.2.234).