The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
9. But there is no origin of that which is (i.e. of Brahman), on account of the impossibility (of such an origin).
Somebody, who has learned from scripture that ether and air, although not in themselves likely to have originated, yet actually are things with a beginning, might feel inclined to suspect that Brahman itself has sprung from something else. And further somebody, who has learned from scripture that from ether and the other elements which are themselves mere effects further effects are produced, might think that also Brahman, from which ether has sprung, is a mere effect.
In order to remove this doubt the Sutra declares that Brahman, whose Self is Being, must not be suspected to have sprung from anything else 'on account of the impossibility.' Brahman which is mere Being cannot spring from mere
being, since the relation of cause and effect cannot exist without a certain superiority (on the part of the cause). Nor again can Brahman spring from that which is something particular, since this would be contrary to experience. For we observe that particular forms of existence are produced from what is general, as, for instance, jars and pots from clay, but not that what is general is produced from particulars. Nor again can Brahman spring from that which is not (asat), for that which is not is without a Self 1, and moreover scripture expressly rejects that view, in the passage 'How could that which is spring from that which is not?' (Kh. Up. VI, 2, 2). Another passage, moreover, expressly denies that Brahman has any progenitor, 'He is the cause, the lord of the lords of the organs, and there is of him neither progenitor nor lord' (Sve. Up. VI, 9).--With regard to ether and air the possibility of an origin has been shown; but in Brahman's case there is no such possibility; hence the cases are not parallel. Nor does the fact of other effects springing from effects imply that Brahman also must be an effect; for the non-admission of a fundamental causal substance would drive us to a retrogressus in infinitum. And that fundamental causal substance which as a matter of fact is generally acknowledged to exist, just that is our Brahman.--Thus there is not any contradiction.
20:1 And cannot therefore constitute a cause; for a cause is the Self of its effects.