Śatapatha Brāhmaṇa ("of a hundred paths") 3.4.4.6

3.4.4.6

tā vā ājyahaviṣo bhavanti | vajro vā ājyametena vai devā vajreṇājyena puraḥ prābhindannimāṃlokānprājayaṃstatho evaiṣa etena vajreṇājyenemāṃlokānprabhinattīmāṃlokānprajayati tasmādājyahaviṣo bhavanti

They have clarified butter for their offering material. For ghee is a thunderbolt, and by that thunderbolt, the ghee, the gods clove the strongholds and conquered these worlds. And so does he cleave these worlds by that thunderbolt, the ghee, and conquer these worlds; therefore they (the Upasads) have ghee for their offering material.