SB 8.24.48

yat-sevayāgner iva rudra-rodanaṁ
 pumān vijahyān malam ātmanas tamaḥ
bhajeta varṇaṁ nijam eṣa so ’vyayo
 bhūyāt sa īśaḥ paramo guror guruḥ
Synonyms: 
yat-sevayā — the Supreme Personality of Godhead, by serving whom; agneḥ — in touch with fire; iva — as it is; rudra-rodanam — a block of silver or gold becomes purified; pumān — a person; vijahyāt — can give up; malam — all the dirty things of material existence; ātmanaḥ — of one’s self; tamaḥ — the mode of ignorance, by which one performs pious and impious activities; bhajeta — may revive; varṇam — his original identity; nijam — one’s own; eṣaḥ — such; saḥ — He; avyayaḥ — inexhaustible; bhūyāt — let Him become; saḥ — He; īśaḥ — the Supreme Personality of Godhead; paramaḥ — the supreme; guroḥ guruḥ — the spiritual master of all other spiritual masters.
Translation: 
One who wants to be free of material entanglement should take to the service of the Supreme Personality of Godhead and give up the contamination of ignorance, involving pious and impious activities. Thus one regains his original identity, just as a block of gold or silver sheds all dirt and becomes purified when treated with fire. May that inexhaustible Supreme Personality of Godhead become our spiritual master, for He is the original spiritual master of all other spiritual masters.
Purport: 

In human life one is meant to undergo austerity to purify one’s existence. Tapo divyaṁ putrakā yena sattvaṁ śuddhyet. Because of contamination by the modes of material nature, one continues in the cycle of birth and death (kāraṇaṁ guṇa-saṅgo ’sya sad-asad-yoni janmasu). Therefore the purpose of human life is to purify oneself of this contamination so that one can regain his spiritual form and not undergo this cycle of birth and death. The recommended process of decontamination is devotional service to the Lord. There are various processes for self-realization, such as karma, jñāna and yoga, but none of them is equal to the process of devotional service. As gold and silver can be freed from all dirty contamination by being put into a fire but not merely by being washed, the living entity can be awakened to his own identity by performing devotional service (yat-sevayā), but not by karma, jñāna or yoga. Cultivation of speculative knowledge or practice of yogic gymnastics will not be helpful.

The word varṇam refers to the luster of one’s original identity. The original luster of gold or silver is brilliant. Similarly, the original luster of the living being, who is part of the sac-cid-ānanda-vigraha, is the luster of ānanda, or pleasure. Ānandamayo bhyāsāt. Every living entity has the right to become ānandamaya, joyful, because he is part of the sac-cid-ānanda-vigraha, Kṛṣṇa. Why should the living being be put into tribulation because of dirty contamination by the material modes of nature? The living entity should become purified and regain his svarūpa, his original identity. This he can do only by devotional service. Therefore, one should adopt the instructions of the Supreme Personality of Godhead, who is described here as guror guruḥ, the spiritual master of all other spiritual masters.

Even though we may not have the fortune to contact the Supreme Lord personally, the Lord’s representative is as good as the Lord Himself because such a representative does not say anything unless it is spoken by the Supreme Personality of Godhead. Śrī Caitanya Mahāprabhu therefore gives a definition of guru. Yāre dekha, tāre kaha ‘kṛṣṇa’-upadeśa: the bona fide guru is he who advises his disciples exactly in accordance with the principles spoken by Kṛṣṇa. The bona fide guru is he who has accepted Kṛṣṇa as guru. This is the guru-paramparā system. The original guru is Vyāsadeva because he is the speaker of Bhagavad-gītā and Śrīmad-Bhāgavatam, wherein everything spoken relates to Kṛṣṇa. Therefore guru-pūjā is known as Vyāsa-pūjā. In the final analysis, the original guru is Kṛṣṇa, His disciple is Nārada, whose disciple is Vyāsa, and in this way we gradually come in touch with the guru-paramparā. One cannot become a guru if he does not know what the Personality of Godhead Kṛṣṇa or His incarnation wants. The mission of the guru is the mission of the Supreme Personality of Godhead: to spread Kṛṣṇa consciousness all over the world.