Śatapatha Brāhmaṇa ("of a hundred paths") 5.1.2.11

5.1.2.11

sa yatsaptadaśa | somagrahāngṛhṇāti saptadaśo vai prajāpatiḥ prajāpatiryajñaḥ sa yāvāneva yajño yāvatyasya mātrā tāvataivāsyaitatsatyaṃ śriyaṃ jyotirujjayati

Now as to why he draws seventeen cups of Soma;--Pragâpati is seventeenfold, Pragâpati is the sacrifice: as great as the sacrifice is, as great as is its measure, with that much he thus wins its truth, its prosperity, its light.