SB 11.10.5

yamān abhīkṣṇaṁ seveta
 niyamān mat-paraḥ kvacit
mad-abhijñaṁ guruṁ śāntam
 upāsīta mad-ātmakam
Synonyms: 
yamān — major regulative principles, such as not to kill; abhīkṣṇam — always; seveta — one should observe; niyamān — minor regulations, such as cleansing the body; mat-paraḥ — one who is devoted to Me; kvacit — as far as possible; mat-abhijñam — one who knows Me as I am in My personal form; gurum — the spiritual master; śāntam — peaceful; upāsīta — one should serve; mat-ātmakam — who is not different from Me.
Translation: 
One who has accepted Me as the supreme goal of life should strictly observe the scriptural injunctions forbidding sinful activities and, as far as possible, should execute the injunctions prescribing minor regulative duties such as cleanliness. Ultimately, however, one should approach a bona fide spiritual master who is full in knowledge of Me as I am, who is peaceful, and who by spiritual elevation is not different from Me.
Purport: 

The word yamān refers to major regulative injunctions necessary for preserving one’s purity. In the Kṛṣṇa consciousness movement all bona fide members must give up eating meat, fish and eggs, and they must also avoid intoxication, gambling and illicit sex. The word abhīkṣṇam indicates that one cannot at any time perform such forbidden activities, even in difficult circumstances. The word niyamān refers to less obligatory injunctions, such as bathing three times daily. In certain difficult situations one may not bathe three times daily yet may still maintain one’s spiritual position. But if one engages in sinful, forbidden activities, even in difficult circumstances, there undoubtedly will be a spiritual falldown. Ultimately, as explained in Upadeśāmṛta, mere adherence to rules and regulations cannot give one spiritual perfection. One must approach a bona fide spiritual master who is mad-abhijñam, or in full knowledge of the personal form of Godhead. The word mat (“Me”) negates the possibility of a bona fide spiritual master having an impersonal conception of the Absolute Truth. Furthermore, the guru must be in complete control of his senses; therefore he is called śānta, or peaceful. Because of being completely surrendered to the mission of the Lord, such a spiritual master is mad-ātmakam, or nondifferent from the Personality of Godhead.