Your are probably looking for Śatapatha Brāhmaṇa 220.127.116.11 (... paśur ha vā eṣa ālabhyate yat puroḍāśaḥ).
Kanda I, adhyaya 2, brahmana 3
THE PREPARATION OF THE ALTAR
5. Thereupon the gods ordained this to be the dakṣiṇā at the new- and full-moon sacrifices, to wit, the Anvāhārya mess of rice , 'lest the oblation should be without a dakṣiṇā.' That (rinsing ...