Śatapatha Brāhmaṇa ("of a hundred paths") 3.4.1.4

3.4.1.4

tadvā anyatara eva vimuktaḥ syāt | anyataro 'vimukto 'tha gṛhṇīyātsa yadanyataro vimuktastenāgato yadvanyataro 'vimuktastenāpacitaḥ

Then only one (of the oxen) is to be unyoked, and the other to be left unyoked; and thereupon he is to take out (the material for offering): for (they argue) in that one of them is unyoked, thereby he (Soma) has arrived; and in that the other is left unyoked, thereby he is honoured.