Śatapatha Brāhmaṇa ("of a hundred paths") 1.1.1.11

1.1.1.11

sa āhavanīyāgāre vaitāṃ rātrīṃ śayīta | gārhapatyāgāre vā devānvā eṣa upāvartate yo vratamupaiti sa yānevopāvarttate teṣāmevaitanmadhye śete 'dhaḥ śayītādhastādiva hi śreyasa upacāraḥ

Let him sleep that night in the house of the Âhavanîya fire or in the house of the Gârhapatya fire. For he who enters on the vow approaches the gods; and he sleeps in the midst of those very gods whom he approaches. Let him sleep on the ground; for from below, as it were, one serves one's superior.
B. THE PREPARATION OF THE OFFERINGS.
After the morning's performance of the Agnihotra and the subsequent rising of the sun, the sacrificer chooses his Brahman, or superintending priest. [In the first place he gets six seats ready, covered with sacrificial grass: two of these, to be used by the Brahman and sacrificer during the ceremony of election, are placed somewhere on the north side of the sacrificial ground; another south of the Âhavanîya fire, to serve for the Brahman's permanent seat (brahmasâdanam), and west of the latter (placed so as to be quite close to the altar to be constructed hereafter, cf. Kâty. Sr. I, 8, 28), the sacrificer's permanent seat; finally a seat north of each of the two fires, the Gârhapatya and the Âhavanîya, to be used by the Adhvaryu on certain occasions. The sacrificer and future Brahman then having seated themselves on the two first-mentioned seats on the north side, the former with his face turned northward, and the latter looking toward east; the sacrificer, holding the wooden sword (sphya) in his left hand, touches the right knee of the Brahman with his right hand, in which he holds barley-corns and] chooses him for his Brahman with the formula: ['Thou, of such and such a family, N.N. Sarman! we are about to perform the Full-moon sacrifice,'] 'O Lord of the earth! Lord of the world! Lord of the great universe! we choose thee for our Brahman!' The chosen one then mutters (cf. Vâg. S. p. 57): 'I am the lord of the earth, I lord of the world, I lord of the great universe (mahâbhûta)! earth! ether! heaven! O God Savitri, thee they choose for their Brahman, their lord of prayer (Brihaspati)!' &c., 'Brihaspati is Brahman to the gods, I to the men!' He (or, according to some, the sacrificer) further says, 'O Lord of speech, protect the sacrifice!' after which he betakes himself to the Brahman's seat (south of the Âhavanîya), and whilst [standing north of it, with his face turned eastward and] looking on it, he mutters: 'Avaunt! unholy one (daidhishavya, lit. son of a re-married woman)! take thee away from here and seat thee on another's seat who is less exalted (pâkatara) than we!' He takes one blade of grass from the seat and flings it towards south-west (the region of the Rakshas or evil spirits) with the formula: 'Expelled is sin with him whom we detest!' and then sits down with the formula 'Here I sit on the seat of Brihaspati, at the command of the divine Savitri! This I proclaim to the fire, this to the wind, this to the earth!' Here he remains seated, with his face towards the Âhavanîya fire, to watch the progress of the ceremony and give directions, whenever he is appealed to. When the full or new-moon sacrifice is performed for the first time, it should be preceded by the Anvârambhanîyâ offering, performed in much the same way as the Paurnamâsî, except that the oblations themselves consist of a rice cake on eleven potsherds for Agni and Vishnu, a potful of boiled (rice) grains (karu) for Sarasvati; and a rice cake on twelve potsherds for Sarasvat; the priest's fee on this occasion consisting of a cow four years old, or a pair of cattle, instead of the Anvâhârya mess. Kâty. Sr. IV, 5, 22-23.