Śatapatha Brāhmaṇa ("of a hundred paths") 14.2.2.44

14.2.2.44

taṃ na prathamayajñe pravṛñjyāt enasyaṃ hi tadatho nenma indraḥ śiraścinadaditi dvitīye vaiva tṛtīye vāpaśīrṣṇā hyevāgre yajñena devā arcantaḥ śrāmyantaścerustasmāddvitīye vaiva tṛtīye vātho tapto vā eṣa śuśucāno bhavati

One must not perform the Pravargya at one's first Soma-sacrifice, since that would be sinful, and lest Indra should cut off his head; but at the second or the third (Soma-sacrifice); for at first the gods went on worshipping and toiling with the headless sacrifice, therefore (he should do so) at the second or the third (sacrifice). Moreover, it will become heated and ablaze;--