Shatapatha Brahmana 10.1.2.8

10.1.2.8

tayorvā etayoḥ pañcadaśasaptadaśayordvātriṃśatstotriyāstato yāḥ pañcaviṃśatiḥ sa pañcaviṃśa ātmātha yāḥ saptātiyanti tāḥ parimādaḥ paśavo haitāḥ paśavaḥ parimāda etāvadvai mahāvrataṃ tadetadatraiva mahāvratamāpnoti

Now these two, the Pañkadasa and Saptadasa, have thirty-two hymn-verses: twenty-five of these are the twenty-five-fold body; and the seven which remain over are the Parimâd (sâmans), for these are the cattle (or animals), (for) cattle are sporting all around us (pari-mâd)--thus much, then, is the Great Rite: thereby he obtains the Great Rite even in this (Agnishtoma).