Shatapatha Brahmana 4.1.5.15

4.1.5.15

tau hocatuḥ | viśīrṣṇā vai yajñena yajadhva iti kathaṃ viśīrṣṇetyupa nu nau hvayadhvamatha vo vakṣyāva iti tatheti tā upāhvayanta tābhyāmetamāśvinaṃ grahamagṛhṇastāvadhvaryū yajñasyābhavatāṃ tāvetadyajñasya śiraḥ pratyadhattāṃ tadadastaddivākīrtyānām brāhmaṇe vyākhyāyate yathā tadyajñasya śiraḥ pratidadhatustasmādeṣa stute bahiṣpavamāne graho gṛhyate stute hi bahiṣpavamāna āgacatām

They said, 'But surely ye worship with a headless sacrifice!'--'How with a headless (sacrifice)?'--'Nay, invite us, and we will tell you!'--'So be it!' so they invited them. They drew this Âsvina cup for them; and those two became the Adhvaryu priests of the sacrifice, and restored the head of the sacrifice. Then, in the chapter of the divâkîrtyas, it is explained how they did restore the head of the sacrifice. Hence this libation is drawn after the chanting of the Bahishpavamâna, for it was after the chanting of the Bahishpavamâna that they arrived.