Śaṅkaraon Brahma Sūtra 3.3.64

The text states that the syllable Om which is a quality, ^ A ‘ thing’ belonging to the A/’g-veda, viz. the pra^ava, is, accord- ing to the AV^andogya-passage, connected with the Sama-veda meditation on the udgitha. Hence meditations also wliich belong to different Vedas may be combined ; for there is no difference between them and things as far as connexion is concerned. — 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 Udgatn sings/ This suggests that, as the abode of the vidy^ (viz. the 0;«kara) is common, the vidyas which abide in it arc common — also. Or else the Sutra may be explained as follows. If the udgitha and so on, which are matters qualifying the sacrificial action, were not all of them common to all sacrificial performances, the vidyas 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 udgitha and so on are common to all performances. As thus the abodes of the vidycis go together, the vidyas abiding in them go together likewise.