Śatapatha Brāhmaṇa ("of a hundred paths") 3.8.3.18

3.8.3.18

gudaṃ tredhā karoti | sthavimopayaḍdbhyo madhyaṃ juhvāṃ dvedhā kṛtvāvadyatyaṇima tryaṅgeṣvathaikacarāyai śroṇeretāvannu juhvāmavadyati

The hind-part he divides into three parts; the broad piece (he reserves) for the by-offerings; the middle one he cuts into the guhû after dividing it in two; the narrow piece (he reserves) for the tryaṅga. Then of the simultaneously moving (right) haunch. This much, then, he cuts into the guhû.