Śatapatha Brāhmaṇa ("of a hundred paths") 2.2.2.1

2.2.2.1

ghnanti vā etadyajñam | yadenaṃ tanvate yannveva rājānamabhiṣuṇvanti tattaṃ ghnanti yatpaśuṃ samjñapayanti viśāsati tattaṃ ghnantyulūkhalamusalābhyāṃ dṛṣadupalābhyāṃ haviryajñaṃ ghnanti

Now, in performing that sacrifice, they slay it; and in pressing out the king (Soma), they slay him; and in quieting and immolating the victim, they slay it. The haviryagña they slay with the mortar and pestle, and with the two mill-stones.