Śatapatha Brāhmaṇa ("of a hundred paths") 1.7.4.10

1.7.4.10

sa yatprāśitramavadyati | yadevātrāviddhaṃ yajñasya yadrudriyaṃ tadevaitannirmimīte 'thāpa upaspṛśati śāntirāpastadadbhiḥ śamayatyatheḍām paśūntsamavadyati

Let him cut off (for the fore-portion) ever so small a piece: thus the dart comes out; let him therefore cut off ever so small a piece. He should (according to some) put clarified butter on one side only, either below or above: thus that which is hard becomes soft and flows forth; and for this reason he should put butter on one side only, either below or above.