Śatapatha Brāhmaṇa ("of a hundred paths") 3.6.1.22

3.6.1.22

atha cadiradhinidadhāti | indrasya cadirasītyaindraṃ hi sado viśvajanasya cāyeti viśvagotrā hyasminbrāhmaṇā āsate tadubhayataścadiṣī upadadhātyuttaratastrīṇi parastrīṇi tāni nava bhavanti trivṛdvai yajño nava vai trivṛttasmānnava bhavanti

He then lays on a mat, with, 'Thou art Indra's mat,'--for the Sadas belongs to Indra,--'a shelter to every one,' for Brâhmans of all families sit therein. He adds two mats, one on each side thereof, and three north of them and three further (to the north): these make nine. For the sacrifice is threefold and nine also is threefold: for this reason there are nine.