Śatapatha Brāhmaṇa ("of a hundred paths") 1.5.3.17

1.5.3.17

saṃgrāmo vā eṣa saṃnidhīyate | yaḥ prayājairyajate yataraṃ vai saṃyattayormitramāgacati sa jayati tadetadupabhṛto 'dhi juhūm mitramāgacati tena prajayati tasmāccaturthe prayāje samānayati barhiṣi

Now, a battle, as it were, is going on here when any one performs the fore-offerings; and whichever of the two combatants a friend (an ally) joins, he obtains the victory: hence a friend thereby joins the guhû from out of the upabhrit, and by him it (or he) obtains the victory. This is why he pours together (butter) at the fourth fore-offering, that to the barhis.