The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
46. And on account of scriptural statement.
'Whatever blessing the priests pray for at the sacrifice, they pray for the good of the sacrificer; thus he said' (Sat. Brâ. I, 3, 1, 26); 'Therefore an udgâtri who knows this may say: what wish shall I obtain for you by my singing?' (Kh. Up. I, 7, 8). These scriptural passages also declare that the fruit of meditations in which the priest is the agent goes to the sacrificer.--All this establishes the conclusion that the meditations on subordinate parts of the sacrifice are the work of the priest.