Śatapatha Brāhmaṇa ("of a hundred paths") 3.5.2.10

3.5.2.10

tadvā etadekaṃ kurvandvayaṃ karoti | yaduttaravediṃ vyāghārayatyatha yaiṣām madhye nābhikeva bhavati tasyai ye pūrve sraktī tayoryā dakṣiṇā

When he pours ghee on the high altar, he does it twofold even while doing it in one. Now that which is the southern of the two front corners of that navel, so to say, which is in the middle of these (sides of the high altar)--