Shatapatha Brahmana 3.6.1.14

3.6.1.14

atha barhīṃṣi | prācīnāgrāṇi codīcīnāgrāṇi cāvastṛṇāti pitṛṣadanamasīti pitṛdevatya vā asyā etadbhavati yannikhātaṃ sā yathānikhātauṣadhiṣu mitā syādevametāsvoṣadhiṣu mitā bhavati

He now strews barhis-grass therein, both eastward-pointed and northward-pointed with, 'Thou art the seat of the Fathers;' for that part of it (the post) which is dug into the ground is sacred to the Fathers; as though it were (naturally) established among plants, and not dug in, so does it become established among those plants.