Śatapatha Brāhmaṇa ("of a hundred paths") 6.1.2.31

6.1.2.31

tadāhuḥ | katarata iṣṭakāyāḥ śira iti yata upaspṛśya yajurvadatītyu haika āhuḥ sa svayamātṛṇāyā evārdhādupaspṛśya yajurvadettatho hāsyaitāḥ sarvāḥ svayamātṛṇāmabhyāvṛttā bhavantīti na tathā kuryādaṅgāni vā asyaitāni parūṃṣi yadiṣṭakā yathā vā aṅge 'ṅge parvanparvañciraḥ kuryāttādṛtkadyo vāva cite 'gnirnidhīyate tadevaitāsāṃ sarvāsāṃ śiraḥ

As to this they say, 'On which side is the head of the brick?'--'Where he touches it and says a prayer,' so say some, 'on one end of the naturally perforated (brick) alone indeed should he say a prayer while touching it, but thus all those (bricks) of his are turned towards the naturally perforated one.' Let him not do so, for those bricks doubtless are his (Agni's) limbs, his joints; and it would be just as if he were to put a head on each limb, on each joint. But indeed, the fire which is deposited on the pile, that is the head of all those (bricks).