Śatapatha Brāhmaṇa ("of a hundred paths") 3.7.1.16

3.7.1.16

sampratyagnimagniṣṭām minoti | yajamāno vā agniṣṭhāgniru vai yajñaḥ sa yadagneragniṣṭhāṃ hvalayeddhvaleddha yajñādyajamānastasmātsampratyagnimagniṣṭhām minotyatha paryūhatyatha paryṛṣatyathāpa upaninayati

That (corner which was) facing the fire he places opposite the fire; for the (corner) facing the fire is the sacrificer, and the sacrifice is fire. Hence were he to turn the fire-corner aside from the fire, the sacrificer would assuredly turn aside from the sacrifice; therefore he places the (corner which was) facing the fire opposite the fire. He then heaps up (earth) round it and presses it firmly all round, and pours water thereon.