Shatapatha Brahmana 5.5.5.9

5.5.5.9

tamubhayeṣāṃ vrīhiyavāṇāṃ gṛhṇāti | vrīhimayamevāgre piṇḍamadhiśrayati tadyajuṣāṃ rūpamatha yavamayaṃ tadṛcāṃ rūpamatha vrīhimayaṃ tatsāmnāṃ rūpa tadetattrayyai vidyāyai rūpaṃ kriyate saiṣā rājasūyayājina udavasānīyeṣṭirbhavati

He prepares it of both rice and barley. He first puts on (the fire) a ball of rice, that being a form (symbol) of the Yagus-formulas; then one of barley, that being a form of the Rik-verses; then one of rice, that being a form of the Sâman-hymns. Thus this is made to be a form of the triple science: and this same (offering) becomes the Udavasânîyâ-ishti (completing oblation) for the performer of the Râgasûya.