Srimad-Bhagavatam: Canto 10 - Chapter 1 - Verse 32
स वै सत्कर्मणां साक्षाद् द्विजातेरिह सम्भव: ।आद्योऽङ्ग यत्राश्रमिणां यथाहं ज्ञानदो गुरु: ॥ ३२ ॥
sa vai sat-karmaṇāṁ sākṣāddvijāter iha sambhavaḥādyo ’ṅga yatrāśramiṇāṁyathāhaṁ jñāna-do guruḥ
My dear friend, he who gives a person his physical birth is his first spiritual master, and he who initiates him as a twice-born brāhmaṇa and engages him in religious duties is indeed more directly his spiritual master. But the person who bestows transcendental knowledge upon the members of all the spiritual orders of society is one’s ultimate spiritual master. Indeed, he is as good as My own self.