Vishnupurana 3.9.4.n2

3.9.4.n2

The text uses the term Dwijáti, which designates a man of the three first castes. The commentator cites various authorities to prove that its sense should be Brahman only, who alone is permitted to enter the fourth order.--'Entrance into the fourth order is never for the Kshatriya and Vaiśya. Entrance into the fourth order is for Brahmans, according to Swayambhu. So says Dattátreya: "Let the Brahman proceed from his dwelling is also the expression of Yama, Samvartta, and Baudháyana."' But this is not the general understanding of the law, nor was it originally so restricted apparently. Manu does not so limit it.