Śatapatha Brāhmaṇa ("of a hundred paths") 1.9.2.16

1.9.2.16

sa yadi prastarasya rūpaṃ kuryāt | vedasyaikaṃ tṛṇamācidyāgraṃ juhvāmanakti madhyaṃ sruve budhnaṃ sthālyām

If he choose to use a substitute for the prastara, he plucks out one stalk from the veda, and anoints its top in the guhû, its middle part in the aruva, and its lower end in the butter-pan.