Śatapatha Brāhmaṇa ("of a hundred paths") 3.4.1.26

3.4.1.26

tadiḍāntam bhavati | nānuyājānyajanti śiro vai yajñasyātithyam pūrvārdho vai śiraḥ pūrvārdhamevaitadyajñasyābhisaṃskaroti sa yaddhānuyājānyajedyathā śīrṣataḥ paryāhṛtya pādau pratidadhyādevaṃ tattasmādiḍāntaṃ bhavati nānuyājānyajanti

It (the guest-offering) ends with the Idâ; no after-offerings are performed. For the guest-offering is the head of the sacrifice, and the head is the fore-part: he thus fits him up as the head of the sacrifice. But were he to perform the after-offerings, it would be as if, by reversing, he were to put the feet in the place of the head. Hence it ends with the Idâ, and no after-offerings are performed.