The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
13. Some also (teach) thus.
The members of one sâkhâ also make a statement about the cognition of non-difference which is preceded by a censure of the perception of difference, 'By the mind alone it is to be perceived, there is in it no diversity. He who perceives therein any diversity goes from death to death' (Bri. Up. IV, 4, 19). Others also ('By knowing the enjoyer, the enjoyed, and the ruler, everything has been declared to be threefold, and this is Brahman,' Svet. Up. I, 12)
record in their text that the entire world, characterised by enjoyers, things to be enjoyed, and a ruler, has Brahman for its true nature.--But as among the scriptural passages referring to Brahman, there are some which represent it as having a form, and others teaching that it is devoid of form, how can it be asserted that Brahman is devoid of form, and not also the contrary?--To this question the next Sûtra replies.