Shatapatha Brahmana 11.1.6.21

11.1.6.21

ta iṣṭvā prācīṃ diśamapaśyan tām prācīmevākurvata seyam prācyeva diktasmādimāḥ prajāḥ prācyaḥ sarpanti prācīṃ hyetāmakurvatopaināmitaḥ kurvīmahīti tāmūrjamakurvatemāṃ khalūrjam paśyemeti sāsau dyaurabhavat

When they had sacrificed they beheld (discovered) the eastern quarter, and made it the eastern (front) quarter; as it now is that eastern (front) quarter: therefore creatures here move in a forward direction, for they (the gods) made that the front quarter. 'Let us improve it from here!' they said, and made it to be strength, 'May we see this strength!' they said; and it became yonder sky.