Śatapatha Brāhmaṇa ("of a hundred paths") 5.1.4.11

5.1.4.11

te vā eta eva trayo yuktā bhavanti | trivṛddhi devānāṃ taddhi devatrādhipraṣṭiyuga eva caturtho 'nveti mānuṣā hi sa taṃ yatra dāsyanbhavati taccaturthamupayujya dadāti tasmādapītarasminyajña eta eva trayo yuktā bhavanti trivṛddhi devānāṃ taddhi devatrādhipraṣṭiyuga eva caturtho 'nveti mānuṣo hi sa taṃ yatra dāsyanbhavati taccaturthamupayujya dadāti

Now only those three (horses) are yoked, for what is threefold belongs to the gods, and this (sacrifice is) with the gods. Alongside the yoke (laid) on the side-horse goes a fourth (horse), for that one is human. When he is about to give that (chariot to the Adhvaryu), he gives it after yoking the fourth (horse) thereto. Hence also at any other sacrifice only those three (horses) are yoked; for what is threefold belongs to the gods, and this (sacrifice is) with the gods. Alongside the yoke of the side-horse goes a fourth (horse), for that one is human. When he is about to give that (chariot) away, he gives it after yoking the fourth (horse) thereto.