tasmad gurum prapadyeta
jijnasuh sreya uttamam
sabde pare ca nisnatam
brahmany upasamasrayam
[SB 11.3.21]

“Any person who seriously desires to achieve real happiness must seek out a bona fide spiritual master and take shelter of him by initiation. The qualification of his spiritual master is that he must have realized the conclusion of the scriptures by deliberation and be able to convince others of these conclusions. Such great personalities, who have taken shelter of the Supreme Godhead, leaving aside all material considerations, are to be understood as bona fide spiritual masters.”

Cited in Srila Prabhupada’s purport to Srimad-Bhagavatam 5.14.41