The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
4. Because speech is preceded by that (viz. fire and the other elements).
Although in the chapter, 'That sent forth fire,' &c., the origin of the prânas is not mentioned, the origin of the three elements, fire, water, and earth only being stated, nevertheless, the fact of the text declaring that speech, prâna, and mind presuppose fire, water, and earth--which in their turn have Brahman for their causal substance--proves that they--and, by parity of reasoning, all prânas--have sprung from Brahman. That speech, prâna, and mind presuppose fire, water, and earth is told in the same chapter, 'For truly, my child, mind consists of earth, breath of water,
speech of fire' (Kh. Up. VI, 5, 4). If their consisting of earth and so on is taken literally, it follows at once that they have sprung from Brahman. And if it be taken in a metaphorical sense only, yet, as the sentence forms part of the chapter which treats of the evolution of names and forms effected by Brahman; and as the introductory phrase runs, 'That by which we hear what is not heard' (Kh. Up. VI, 1, 3); and as the concluding passage is 'In it all that exists has its Self' (Kh. Up. VI, 8, 7); and as the matter is moreover known from other scriptural passages; we understand that also the statement about mind and so on consisting of earth, &c., is meant to teach that they are products of Brahman.--It is therefore an established conclusion that the prânas also are effects of Brahman.