Shatapatha Brahmana 14.1.1.24

14.1.1.24

tau hopaninye tau yadopaninye'thāsya śiraścittvānyatrāpanidadhaturathāśvasya śira āhṛtya taddhāsya pratidadhatustena hābhyāmanūvāca sa yadābhyāmanūvācāthāsya tadindraḥ śiraścicedāthāsya svaṃ śira āhṛtya taddhāsya pratidadhatuḥ

He then received them (as his pupils); and when he had received them, they cut off his head, and put it aside elsewhere; and having fetched the head of a horse, they put it on him: therewith he taught them; and when he had taught them, Indra cut off that head of his; and having fetched his own head, they put it on him again,