Śatapatha Brāhmaṇa ("of a hundred paths") 10.2.3.1

10.2.3.1

yā vā iyaṃ vediḥ saptavidhasya eṣā vedermātrā sa devayajanamadhyavasāya pūrvayā dvārā patnīśālam prapadya gārhapatyāyoddhatyāvokṣati gārhapatyasyoddhatātsapta prācaḥ prakramānprakrāmati tataḥ prāñcaṃ vyāmaṃ vimimīte tasya madhya āhavanīyāyoddhatyāvokṣati pūrvārdhādvyāmasya trīnprācaḥ prakramānprakrāmati sa vedyantaḥ

Now this Vedi (altar-ground, viz. the Mahâvedi of the Soma-sacrifice) is just that (right) measure for the Vedi of the sevenfold (fire-altar). Having fixed upon (the place for) the sacrificial ground, he enters the Patnîsâla by the front (east) door, and having thrown up (the ground) for the Gârhapatya, he sprinkles it with water. From the raised (site) of the Gârhapatya he strides seven steps eastward. From there he measures off a fathom towards the east, and having, in the middle thereof, thrown up (the ground) for the Âhavanîya, he sprinkles it with water. From the front part of the fathom he strides three steps eastward: that is the end of the Vedi.