III, 3, 64
The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut [1896] at sacred-texts.com
64. And because the text states a quality (of the vidyâ) to be common (to the three Vedas).
The text states that the syllable Om which is a quality,
i.e. the abode of a meditation, is common to the three Vedas, 'By that syllable the threefold knowledge proceeds. With Om the Adhvaryu gives orders, with Om the Hotri recites, with Om the Udgâtri sings.' This suggests that, as the abode of the vidyâ (viz. the Omkâra) is common, the vidyâs which abide in it are common also.--Or else the Sûtra may be explained as follows. If the udgîtha and so on, which are matters qualifying the sacrificial action, were not all of them common to all sacrificial performances, the vidyâs resting on them would not go together. But the scriptural passages which teach the sacrificial performances and extend over all subordinate matters, state that the udgîtha and so on are common to all performances. As thus the abodes of the vidyâs go together, the vidyâs abiding in them go together likewise.