The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut [1890] at sacred-texts.com
15. Moreover (the ânandamaya is Brahman because)
the same (Brahman) which had been referred to in the mantra is sung, (i.e. proclaimed in the Brâhmana passage as the ânandamaya.)
The Self, consisting of joy, is the highest Brahman for the following reason also 1. On the introductory words 'he who knows Brahman attains the highest' (Taitt. Up. II, 1), there follows a mantra proclaiming that Brahman, which forms the general topic of the chapter, possesses the qualities of true existence, intelligence, infinity; after that it is said that from Brahman there sprang at first the ether and then all other moving and non-moving things, and that, entering into the beings which it had emitted, Brahman stays in the recess, inmost of all; thereupon, for its better comprehension, the series of the different Selfs ('different from this is the inner Self,' &c.) are enumerated, and then finally the same Brahman which the mantra had proclaimed, is again proclaimed in the passage under discussion, 'different from this is the other inner Self, which consists of bliss.' To assume that a mantra and the Brâhmana passage belonging to it have the same sense is only proper, on account of the absence of contradiction (which results therefrom); for otherwise we should be driven to the unwelcome inference that the text drops the topic once started, and turns to an altogether new subject.
Nor is there mentioned a further inner Self different from the Self consisting of bliss, as in the case of the Self consisting of food, &c. 2 On the same (i.e. the Self consisting of bliss) is founded, 'This same knowledge of Bhrigu and Varuna; he understood that bliss is Brahman' (Taitt. Up. III, 6). Therefore the Self consisting of bliss is the highest Self.
68:1 The previous proofs were founded on linga; the argument which is now propounded is founded on prakarana.
68:2 While, in the case of the Selfs consisting of food and so on, a further inner Self is duly mentioned each time. It cannot, therefore, be concluded that the Selfs consisting of food, &c., are likewise identical with the highest Self referred to in the mantra.