The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
31. Beyond (Brahman, there is something) further, on account of the designations of bank, measure, connexion, separation.
With reference to this Brahman which we have ascertained to be free from all plurality there now arises the doubt--due to the conflicting nature of various scriptural statements--whether something exists beyond it or not. We therefore enter on the task of explaining the true meaning of those scriptural passages which seem to indicate that there is some entity beyond, i.e. apart from Brahman.
The pûrvapakshin maintains that some entity must be admitted apart from Brahman, because Brahman is spoken of as being a bank; as having size; as being connected; as being separated.--As a bank it is spoken of in the passage, Kh. Up. VIII, 4, 1, 'That Self is a bank, a boundary.' The word 'bank' (setu) ordinarily denotes a structure of earth, wood and the like, serving the purpose of checking the flow of water. Here, being applied to the Self, it intimates that there exists something apart from the Self, just as there exists something different from an ordinary bank. The same conclusion is confirmed by the words, 'Having passed the bank' (VIII, 4, 2). For as in ordinary life a man after having crossed a bank reaches some place which is not a bank, let us say a forest; so, we must understand, a man after having crossed, i.e. passed beyond the Self reaches something which is not the Self.--As having size Brahman is spoken of in the following passages, 'This Brahman has four feet (quarters), eight hoofs, sixteen parts.' Now it is well known from ordinary experience that wherever an object, a coin, e.g. has a definite limited size, there exists something different from that object; we therefore must assume that there also
exists something different from Brahman.--Brahman is declared to be connected in the following passages, 'Then he is united with the True' (Kh. Up. VI, 8, 1), and 'The embodied Self is embraced by the highest Self (Bri. Up. IV, 3, 21). Now we observe that non-measured things are connected with things measured, men, e.g. with a town. And scripture declares that the individual souls are, in the state of deep sleep, connected with Brahman. Hence we conclude that beyond Brahman there is something unmeasured.--The same conclusion is finally confirmed by those texts which proclaim difference, so e.g. the passage, I, 6, 6 ff. ('Now that golden person who is seen within the sun' &c.), which at first refers to a Lord residing in the sun and then mentions a Lord residing in the eye, distinct from the former ('Now the person who is seen within the eye'). The text distinctly transfers to the latter the form &c. of the former 1 ('The form of that person is the same as the form of the other' &c.), and moreover declares that the lordly power of both is limited, 'He obtains through the one the worlds beyond that and the wishes of the devas' &c.; which is very much as if one should say, 'This is the reign of the king of Magadha and that the reign of the king of Videha.'
From all this it follows that there exists something different from Brahman.