Shatapatha Brahmana 14.1.1.23

14.1.1.23

tau hocatuḥ āvām tvā tasmāttrāsyāvaha iti katham mā trāsyethe iti yadā nā upaneṣyase'tha te śiraścittvānyatrāpanidhāsyāvo'thāśvasya śira āhṛtya tatte pratidhāsyāvastena nāvanuvakṣyasi sa yadā nāvanuvakṣyasyatha te śiraścetsyatyatha te svaṃ śira āhṛtya tatte pratidhāsyāva iti tatheti

They said, 'We two shall protect thee from him.'--'How will ye protect me?' he replied.--They said, 'When thou wilt have received us as thy pupils, we shall cut off thy head and put it aside elsewhere; then we shall fetch the head of a horse, and put it on thee: therewith thou wilt teach us; and when thou wilt have taught us, then Indra will cut off that head of thine; and we shall fetch thine own head, and put it on thee again.'--'So be it,' he replied.