Shatapatha Brahmana 9.5.1.57

9.5.1.57

tasyai vājinena caranti | tasmindakṣiṇāṃ dadhāti tūparau mithunau dadyādityabhyājñāyenaiva manya iti ha smāha māhitthiḥ sravatyu haiṣāgnicita āhutiḥ somāhutiryāmaniṣṭake juhoti

They proceed with the whey of that (dish of clotted curds). At this (oblation of whey) he gives a dakshinâ (sacrificial gift): 'Let him give a pair of hornless he-goats,' so (they say);--'Only by assignment, I think:' said Mâhitthi. And, verily, this libation of the fire-builder flows away as a libation of Soma which one offers on a (fire) without bricks.