Śaṅkaraon Brahma Sūtra 3.3.63

The passage, ‘From the seat of the Hotrf he sets right any mistake committed in the udgitha’ (AV^. Up. I, 5, 5), declares that, owing to the might of the meditation on the unity of pra;^ava and udgitha, the Hotrf sets right any mistake he may commit in his work, by means of the work of the Hotr/. Now, as a meditation mentioned in one Veda is con- nected (with what is mentioned in another Veda) in the same way as a thing mentioned in another Veda, the above passage suggests the conclusion that all meditations — on members of sacrificial acts—in whatever Veda they may be mentioned have to be combined