III, 3, 64

64. And because the text states a quality (of the vidyā) 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 Hotṛ recites, with Om the Udgātṛ sings.' This suggests that, as the abode of the vidyā (viz. the Oṃkāra) is common, the vidyās which abide in it are common also.--Or else the Sūtra may be explained as follows. If the udgītha and so on, which are matters qualifying the sacrificial action, were not all of them common to all sacrificial performances, the vidyās 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 udgītha and so on are common to all performances. As thus the abodes of the vidyās go together, the vidyās abiding in them go together likewise.