Śatapatha Brāhmaṇa ("of a hundred paths") 1.7.4.11

1.7.4.11

sa vai yāvanmātramivaivāvadyet | tathā śalpaḥ pracyavate tasmādyāvanmātramivaivāvadyedanyatarata ājyaṃ kuryādadhastādvopariṣṭādvā tathā khadanniḥsaraṇavadbhavati tathā nisravati tasmādanyatarata ājyaṃ kuryādadhastādvopariṣṭādvā

Having (nevertheless) made an underlayer of butter and a double cutting from the oblation (havis), he pours butter on the upper side of it; for it is only in this way that this becomes part of the sacrifice.