Śatapatha Brāhmaṇa ("of a hundred paths") 1.4.4.7

1.4.4.7

āsīnastamāghārayati | yam manasa āghārayati tiṣṭhṃstaṃ yaṃ vāce manaśca ha vai vākca yujau devebhyo yajñaṃ vahato yataro vai yujorhrasīyānbhavatyupavahaṃ vai tasmai kurvanti vāgvai manaso hrasīyasyaparimitataramiva hi manaḥ parimitatareva hi vāktadvāca evaitadupavahaṃ karoti te sayujau devebhyo yajñaṃ vahatastasmāttiṣṭhanvāca āghārayati

Sitting he makes that libation which he makes for the mind, and standing that which he makes for speech. Mind and speech, when yoked together, assuredly convey the sacrifice to the gods. But when one of two yoke-fellows is smaller (than the other) they give him a shoulder-piece. Now speech is indeed smaller than mind; for mind is by far the more unlimited, and speech is by far the more limited (of the two); hence he thereby (by standing) gives a shoulder-piece to speech, and as well-matched yoke-fellows these two now convey the sacrifice to the gods: for speech, therefore, he sprinkles while standing.