Śatapatha Brāhmaṇa ("of a hundred paths") 3.9.2.17

3.9.2.17

tāni vā etāni sapta yajūṃṣi bhavanti | caturbhirgṛhṇātyekena jaghanena gārhapatyaṃ sādayatyekena pariharatyekenāgnīdhre tāni sapta yatra vai vācaḥ prajātāni candāṃsi saptapadā vai teṣām parārdhyā śakvaryetāmabhisampadaṃ tasmātsapta yajūṃṣi bhavanti

Now there are here seven formulas; with four he takes (the water), with one he puts it down behind the Gârhapatya, with one he carries it round, with one (he puts it down) in the Âgnîdhra,--this makes seven. For when the metres were produced from Vâk (speech), the one consisting of seven feet, the Sakvarî, was the last (highest) of them;--that completeness (he brings about): hence there are seven formulas.