The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
56. Or else there is no contradiction (implied in our opinion); as in the case of mantras and the like.
Or else we may put the matter as follows. There is no reason whatever to suspect a contradiction if we declare certain vidyâs enjoined in one Sâkhâ to be valid for the udgîtha and so on belonging to other Sâkhâs also; for there is no more room for contradiction than in the case of mantras. We observe that mantras, acts, and qualities of acts which are enjoined in one Sâkhâ are taken over by other Sâkhâs also. So e.g. the members of certain Yagur-veda Sâkhâs do not exhibit in their text the mantra, 'Thou art the kutaru 1', which accompanies the taking of the stone (with which the rice-grains are ground); all the same we meet in their text with the following injunction of application, 'Thou art the cock, with this mantra he takes the stone; or else with the mantra, Thou art the kutaru.'
Again, the text of some Sâkhâ does not contain a direct injunction of the five offerings called prayâgas which are made to the fuel and so on, but it contains the injunction of secondary matters connected with the prayâgas, viz. in the passage, 'the seasons indeed are the prayâgas; they are to be offered in one and the same spot 1.'--Again, the text of some Sâkhâ does not contain an injunction as to the species of the animal to be sacrificed to Agnîshomau--such as would be 'a he-goat is sacrificed to Agnîshomau 2;'--but in the same Sâkhâ we meet with a mantra which contains the required specification, 'Hotri', recite the anuvâkyâ, for the fat of the omentum of the he-goat 3.' Similarly mantras enjoined in one Veda only, such as 'O Agni, promote the hautra, promote the sacrifice,' are seen to be taken over into other Vedas also. Another example (of the transference of mantras) is supplied by the hymn, 'He who as soon as born showed himself intelligent,' &c. (Rik. Samh. II, 12), which although read in the text of the Bahvrikas is employed in the Taittirîya Veda also, according to Taitt. Samh. VII, 5, 5, 2, 'The Saganîya hymn is to be recited.'--Just as, therefore, the members of sacrificial actions on which certain vidyâs rest are valid everywhere, so the vidyâs themselves also which rest on those members are valid for all Sâkhâs and Vedas.
273:1 Maitrâyanîya Samhitâ I, 1, 6.