SB 4.17.5

sanat-kumārād bhagavato
 brahman brahma-vid-uttamāt
labdhvā jñānaṁ sa-vijñānaṁ
 rājarṣiḥ kāṁ gatiṁ gataḥ
Word for word: 
sanat-kumārāt — from Sanat-kumāra; bhagavataḥ — the most powerful; brahman — my dear brāhmaṇa; brahma-vit-uttamāt — well versed in the Vedic knowledge; labdhvā — after achieving; jñānam — knowledge; sa-vijñānam — for practical application; rāja-ṛṣiḥ — the great saintly King; kām — which; gatim — destination; gataḥ — achieved.
Translation: 
The great saintly King, Mahārāja Pṛthu, received knowledge from Sanat-kumāra, who was the greatest Vedic scholar. After receiving knowledge to be applied practically in his life, how did the saintly King attain his desired destination?
Purport: 

There are four Vaiṣṇava sampradāyas (systems) of disciplic succession. One sampradāya comes from Lord Brahmā, one from the goddess of fortune, one from the Kumāras, headed by Sanat-kumāra, and one from Lord Śiva. These four systems of disciplic succession are still going on. As King Pṛthu has illustrated, one who is serious about receiving transcendental Vedic knowledge must accept a guru, or spiritual master, in one of these four disciplic successions. It is said that unless one accepts a mantra from one of these sampradāyas, the so-called mantra will not act in Kali-yuga. Many sampradāyas have sprung up without authority, and they are misleading the people by giving unauthorized mantras. The rascals of these so-called sampradāyas do not observe the Vedic rules and regulations. Although they are addicted to all kinds of sinful activities, they still offer the people mantras and thus mislead them. Intelligent persons, however, know that such mantras will never be successful, and as such they never patronize such upstart spiritual groups. People should be very careful of these nonsensical sampradāyas. To get some facility for sense gratification, unfortunate people in this age receive mantras from these so-called sampradāyas. Pṛthu Mahārāja, however, showed by his example that one should receive knowledge from a bona fide sampradāya. Therefore Mahārāja Pṛthu accepted Sanat-kumāra as his spiritual master.