Where is this verse found "The knower of Brahman is a Brahmin"
स यो ह वै तत् परमं ब्रह्म वेद ब्रह्मैव भवति नास्याब्रह्मवित्कुले भवति । तरति शोकं तरति पाप्मानं गुहाग्रन्थिभ्यो विमुक्तोऽमृतो भवति ॥९॥"
III-ii-9: Anyone who knows that supreme Brahman becomes Brahman indeed. In his line is not born anyone who does not know Brahman. He overcomes grief, and rises above aberrations; and becoming freed from the knots of the heart, he attains immortality.
Sri Skanda Purana Vol.18 Book VI , Nagar Kanda , Chapter 239 , ( Efficacy of Adoration , Penance etc.) Verse 31-34.:
जन्मना जायते शूद्रः संस्कारात् द्विज उच्यते। शापानुग्रहसामर्थ्यं तथा क्रोधः प्रसन्नता ।|31-34||
31 : A Man is no better than a sudra at his birth .He is called Brahmana (Twice Born) due to the consecration. The ability to curse and to bless, the state of being angry and pleased and the status of being the foremost in all three worlds occur only in Brahmana.
जन्मना जायते शूद्रः संस्कारात् द्विज उच्यते। वेदाभ्यास भवति विप्र ब्रह्मम जानति ब्राह्मणः||
A Person is born Shudra by birth and becomes Brahmin after ritual. By the practice of the Vedas, he becomes wise and by knowing the Brahman he becomes Brahman himself.
|| Om Tat Sat ||