SB 11.17.32

agnau gurāv ātmani ca
 sarva-bhūteṣu māṁ param
apṛthag-dhīr upasīta
 brahma-varcasvy akalmaṣaḥ
Synonyms: 
agnau — in fire; gurau — in the spiritual master; ātmani — in oneself; ca — also; sarva-bhūteṣu — in all living entities; mām — Me; param — the Supreme; apṛthak-dhīḥ — without any concept of duality; upāsīta — one should worship; brahma-varcasvī — possessing Vedic enlightenment; akalmaṣaḥ — sinless.
Translation: 
Thus enlightened in Vedic knowledge by service to the spiritual master, freed from all sins and duality, one should worship Me as the Supersoul, as I appear within fire, the spiritual master, one’s own self and all living entities.
Purport: 

One becomes glorious and enlightened by faithfully serving a bona fide spiritual master, who is expert in the Vedic way of life. Thus purified, one never engages in sinful activities, which immediately extinguish the fire of spiritual enlightenment; nor does one become foolish and small-minded, trying to exploit material nature for personal sense gratification. A purified human being is apṛthag-dhī, or without consciousness of duality, because he has been trained to observe the Supreme Personality of Godhead within all things. Such sublime consciousness should be systematically taught throughout the world so that human society will become peaceful and sublime.