The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut [1896] at sacred-texts.com
4. (Scripture) also declares this.
The Veda also declares the identity of the vidyâs; for all Vedânta-texts represent the object of knowledge as one; cp. e.g. Kâ. Up. I, 2, 15, 'That word which all the Vedas record;' Ait. Âr. Ill, 2,3,12, 'Him only the Bahvrikas consider in the great hymn, the Adhvaryus in the sacrificial fire, the Khandogas in the Mahâvrata ceremony.'--To quote some other instances proving the unity of the vidyâs: Kâ. Up. I, 6, 2, mentions as one of the Lord's qualities that he
causes fear; now this very same quality is referred to in the Taitt. Up. II, 7, in order to intimate disapprobation of those who are opposed to the absolute unity of that which is, 'For if he makes but the smallest distinction in it (the Self), there is fear for him. But that fear is only for him who knows (a difference) and does not know (the oneness).'--Similarly the Vaisvânara, who in the Vâgasaneyaka is imaginatively represented as a span long, is referred to in the Khândogya as something well known, 'But he who worships that Vaisvânara Self which is a span long,' &c. (Kh. Up. V, 18, 1).
And as, on the ground of all Vedânta-texts intimating the same matters, hymns and the like which are enjoined in one place are employed in other places (where they are not expressly enjoined) for the purposes of devout meditation, it follows that all Vedânta-texts intimate also (identical) devout meditations.