SB 1.5 - Receiving the mantra from guru
One who worships the yajña-puruṣa, Lord Viṣṇu, who has no material form (amūrtikam), in the form of transcendental sound vibration (mantra mūrtim) is the actual seer. One should worship the Lord, who is the subject of the dhyāna-mantra with the 16 items of worship while uttering vāsudevāya namaḥ. Anything which is beyond the scope of experience by our imperfect senses can be realized fully by the sound representation. This is a factual experience of the transcendental Personality of Godhead, who possesses a pure form of sac-cid-ānanda. The transcendental form of the Lord can be experienced without difficulty (amūrtikam – mūrti also means difficulty) by our original spiritual senses which can be revived by chanting the holy mantras. Such sound should be received from the transparent agency of the bona fide guru, and the chanting may be practiced by the direction of the spiritual master. This method of worship is recommended in the pāñcarātrika system, and this is more important than the Vedānta for this modern age. One who worships the Lord possesses complete scriptural conclusion. This does not refer to scriptures devoid of bhakti, which are not wholesome but deficient as confirmed in SB 1.5.8 where Nārada said that Vyāsa’s writing of Vedānta was insufficient because he was never satisfied.
Thus by Lord Kṛṣṇa, Nārada was endowed first with transcendental knowledge of the Lord, then with the spiritual opulences like aṇima and other siddhis, and then with His intimate loving service (prema). By chanting the holy names of the Lord without offenses, one can understand the Lord’s existence in the transcendental realm. The Lord reveals His identity gradually to one who has unflinching faith in guru and Kṛṣṇa. After this, the devotee is endowed with mystic opulences, and above all he is accepted in the confidential entourage of the Lord and is entrusted with specific service of the Lord thru the agency of the spiritual master. Vyāsa then learnt the mantra from Nārada.