Śatapatha Brāhmaṇa ("of a hundred paths") 2.5.1.18

2.5.1.18

athāta āvṛdeva | nopakirantyuttaravediṃ visṛṣṭamasatsarvamasadvaiśvadevamasaditi tredhā barhiḥ saṃnaddham bhavati tatpunarekadhaitaddhi prajananasya rūpam prajananamu hīdam pitā mātā yajjāyate tattṛtīyaṃ tasmāttredhā satpunarekadhā prasva upasaṃnaddhā bhavanti tam prastaraṃ gṛhṇāti prajananamu hīdam prajananamu hi prasvastasmātprasūḥ prastaraṃ gṛhṇāti

Now as to the course of proceeding. They do not raise an uttara-vedi in order that it (the sacred work) may be unobstructed, that it may be entire, that it may be (worthy) of the All-gods.--The barhis is tied up in three (bunches), and then again in one; for such is the characteristic form of generation, since father and mother are a productive (pair), and what is born forms a third element: hence that which is threefold is again (made) one. Thereto flowering shoots (of sacrificial grass) are tied: these he uses for the prastara; for this is a productive union, and productive indeed are flowering shoots: this is why he takes flowering shoots for the prastara.