The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
18. But Gaimini thinks that (the reference to the individual soul) has another purport, on account of
the question and answer; and thus some also (read in their text).
Whether the passage under discussion is concerned with the individual soul or with Brahman, is, in the opinion of the teacher Gaimini, no matter for dispute, since the reference to the individual soul has a different purport, i.e. aims at intimating Brahman. He founds this his opinion on a question and a reply met with in the text. After Agâtasatru has taught Bâlâki, by waking the sleeping man, that the soul is different from the vital air, he asks the following question, 'Bâlâki, where did this person here sleep? Where was he? Whence came he thus back?' This question clearly refers to something different from the individual soul. And so likewise does the reply, 'When sleeping he sees no dream, then he becomes one with that prâna alone;' and, 'From that Self all prânas proceed, each towards its place, from the prânas the gods, from the gods the worlds.'--Now it is the general Vedânta doctrine that at the time of deep sleep the soul becomes one with the highest Brahman, and that from the highest Brahman the whole world proceeds, inclusive of prâna, and so on. When Scripture therefore represents as the object of knowledge that in which there takes place the deep sleep of the soul, characterised by absence of consciousness and utter tranquillity, i.e. a state devoid of all those specific cognitions which are produced by the limiting adjuncts of the soul, and from which the soul returns when the sleep is broken; we understand that the highest Self is meant.--Moreover, the Vâgasaneyisâkhâ, which likewise contains the colloquy of Bâlâki and Agâtasatru, clearly refers to the individual soul by means of the term, 'the person consisting of cognition' (vigñânamaya), and distinguishes from it the highest Self ('Where was then the person consisting of cognition? and from whence did he thus come back?' Bri. Up. II, 1, 16); and later on, in the reply to the above question, declares that 'the person consisting of cognition lies in the ether within the heart.' Now we know that the word 'ether' may be used to denote the highest Self, as, for instance, in
the passage about the small ether within the lotus of the heart (Kh. Up. VIII, 1, 1). Further on the Bri. Up. says, 'All the Selfs came forth from that Self;' by which statement of the coming forth of all the conditioned Selfs it intimates that the highest Self is the one general cause.--The doctrine conveyed by the rousing of the sleeping person, viz. that the individual soul is different from the vital air, furnishes at the same time a further argument against the opinion that the passage under discussion refers to the vital air.