Shatapatha Brahmana 10.2.1.9

10.2.1.9

atha pañcamīṃ citimupadhāya tredhāgniṃ vimimīte sa madhyame vitṛtīye sahasramṛjvālikhitā iṣṭakā upadadhāti tadyānīmāni vayasaḥ pratyañci śīrṣṇa ā pucādṛjūni lomāni tāni tatkaroti

And when he has laid down the fifth layer, he measures out the altar in three parts, and on the central part he places the one thousand bricks marked with straight lines: he thereby gives to it those straight plumes of the bird pointing backwards (with their tops, and covering it) from head to tail.