Śaṅkaraon Brahma Sūtra 1.3.37
Advaita Vedānta
Sūtra
तदभावनिर्धारणे च प्रवृत्तेः
tadabhāvanirdhāraṇe ca pravṛtteḥ
Sūtra translation
And on account of (Gautama) proceeding (to initiate Jâbâla) on the ascertainment of (his) not being that (i.e. a Sûdra).
The Sûdras are not qualified for that reason also that Gautama, having ascertained Jâbâla not to be a Sûdra from his speaking the truth, proceeded to initiate and instruct him. 'None who is not a brâhmana would thus speak out. Go and fetch fuel, friend, I shall initiate you. You have not swerved from the truth' ( IV, 4, 5); which scriptural passage furnishes an inferential sign (of the Sûdras not being capable of initiation).