Śatapatha Brāhmaṇa ("of a hundred paths") 14.2.2.49

14.2.2.49

atho āhuḥ yajñasya śīrṣacinnasya śira etaddevāḥ pratyadadhuryadātithyaṃ na ha vā asyāpaśīrṣṇā kena cana yajñeneṣṭam bhavati ya evametadveda

And they also say,--when the sacrifice had its head cut off, the gods on that occasion restored it as the hospitable reception (of King Soma), and verily for him who so knows this offering is not made with any headless sacrifice whatever.