Chandogya Upanishad
Prapathaka IV, Khanda 16
Verily, he who purifies (Vâyu) is the sacrifice, for he (the air) moving along purifies everything. Because moving along he purifies everything, therefore he is the sacrifice. Of that sacrifice there are two ways, by mind and by speech.
The Brahman priest performs one of them in his mind , the Hotri, Adhvaryu, and Udgâtri priests perform the other by words. When the Brahman priest, after the Prâtaranuvâka ceremony has begun, but before the recitation of the Paridhânîyâ hymn, has (to break his silence and) to speak,
He performs perfectly the one way only (that by words), but the other is injured. As a man walking on one foot, or a carriage going on one wheel, is injured, his sacrifice is injured, and with the injured sacrifice the sacrificer is injured; yes, having sacrificed, he becomes worse.
But when after the Prâtaranuvâka ceremony has begun, and before the recitation of the Paridhânîyâ hymn, the Brahman priest has not (to break his silence and) to speak, they perform both ways perfectly, and neither of them is injured.