Śatapatha Brāhmaṇa ("of a hundred paths") 7.3.1.44

7.3.1.44

tā etā yajuṣmatya iṣṭakāḥ | tā ātmannevopadadhāti na pakṣapuceṣvātmanhyeva yajuṣmatya iṣṭakā upadhīyante na pakṣapuceṣu na sādayati nedretaḥ prajātiṃ sthāpayānīti

This same (sand represents) bricks with special prayers: he places it on the body (of the altar), not on the wings and tail; for bricks with special prayers are placed on the body, not on the wings and tail. He does not 'settle' it, lest he should stop the seed, and generation.