I, 3, 37
The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut [1890] at sacred-texts.com
37. And on account of (Gautama) proceeding (to initiate Gâbâla) 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 Gâbâla 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' (Kh. Up. IV, 4, 5); which scriptural passage furnishes an inferential sign (of the Sûdras not being capable of initiation).