The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
28. Prâna (breath) is Brahman, that being understood from a connected consideration (of the passages referring to prâna).
In the Kaushîtaki-brâhmana-upanishad there is recorded a legend of Indra and Pratardana which begins with the words, 'Pratardana, forsooth, the son of Divodâsa came by means of fighting and strength to the beloved abode of Indra' (Kau. Up. III, 1). In this legend we read: 'He said: I am prâna, the intelligent Self (pragñâtman), meditate on me as Life, as Immortality' (III, 2). And later on (III, 3), 'Prâna alone, the intelligent Self, having laid hold of this body, makes it rise up.' Then, again (III, 8), 'Let no man try to find out what speech is, let him know the speaker.' And in the end (III, 8), 'That breath indeed is the intelligent Self, bliss, imperishable, immortal.'--Here the doubt presents itself whether the word prâna denotes merely breath, the modification of air, or the Self of some divinity, or the individual soul, or the highest Brahman.--But, it will be said at the outset, the Sûtra I, 1, 21 already has shown that the word prâna refers to Brahman, and as here also we meet with characteristic marks of Brahman, viz. the words 'bliss, imperishable, immortal,' what reason is there for again raising the same doubt?--We reply: Because there are observed here characteristic marks of different kinds. For in the legend we meet not only with marks indicating Brahman, but also with marks pointing to other beings Thus Indra's words, 'Know me only' (III, 1) point to the Self of a divinity; the words, 'Having laid hold of this body it makes it rise up,' point to the breath; the words, 'Let no man try to find out what speech is, let him know
the speaker,' point to the individual soul. There is thus room for doubt.
If, now, the pûrvapakshin maintains that the term prâna here denotes the well-known modification of air, i.e. breath, we, on our side, assert that the word prâna must be understood to denote Brahman.--For what reason?--On account of such being the consecutive meaning of the passages. For if we examine the connexion of the entire section which treats of the prâna, we observe that all the single passages can be construed into a whole only if they are viewed as referring to Brahman. At the beginning of the legend Pratardana, having been allowed by Indra to choose a boon, mentions the highest good of man, which he selects for his boon, in the following words, 'Do you yourself choose that boon for me which you deem most beneficial for a man.' Now, as later on prâna is declared to be what is most beneficial for man, what should prâna denote but the highest Self? For apart from the cognition of that Self a man cannot possibly attain what is most beneficial for him, as many scriptural passages declare. Compare, for instance, Sve. Up. III, 8, 'A man who knows him passes over death; there is no other path to go.' Again, the further passage, 'He who knows me thus by no deed of his is his life harmed, not by theft, not by bhrûnahatyâ' (III, 1), has a meaning only if Brahman is supposed to be the object of knowledge. For, that subsequently to the cognition of Brahman all works and their effects entirely cease, is well known from scriptural passages, such as the following, 'All works perish when he has been beheld who is the higher and the lower' (Mu. Up. II, 2, 8). Moreover, prâna can be identified with the intelligent Self only if it is Brahman. For the air which is non-intelligent can clearly not be the intelligent Self. Those characteristic marks, again, which are mentioned in the concluding passage (viz. those intimated by the words 'bliss,' 'imperishable,' 'immortal') can, if taken in their full sense, not be reconciled with any being except Brahman. There are, moreover, the following passages, 'He does not increase by a good action, nor decrease by a bad action. For he makes him whom he wishes
to lead up from these worlds do a good deed; and the same makes him whom he wishes to lead down from these worlds do a bad deed;' and, 'He is the guardian of the world, he is the king of the world, he is the Lord of the world' (Kau. Up. III, 8). All this can be properly understood only if the highest Brahman is acknowledged to be the subject-matter of the whole chapter, not if the vital air is substituted in its place. Hence the word prâna denotes Brahman.