Shatapatha Brahmana 9.5.1.43

9.5.1.43

yadveva vaiśvakarmaṇāni juhoti | prāyaṇaṃ ca hāgnerudayanaṃ ca sāvitrāṇi prāyaṇaṃ vaiśvakarmaṇānyudayanaṃ sa yatsāvitrāṇyeva juhuyānna vaiśvakarmaṇāni yathā prāyaṇameva kuryānnodayanaṃ tādṛktadatha yadvaiśvakarmaṇānyeva juhuyānna sāvitrāṇi yathodayanameva kuryānna prāyaṇaṃ tādṛktadubhayāni juhoti prāyaṇaṃ ca tadudayanaṃcakaroti

And, again, as to why he offers the Vaisvakarmana (oblations). For the fire-altar there is both a beginning and an end: the Sâvitra (formulas) are the beginning, and the Vaisvakarmana (oblations) the end. Were he to offer only those to Savitri, and not those to Visvakarman, it would be as if he made only a beginning and no end; and were he to offer only those to Visvakarman, and not those to Savitri, it would be as if he made only an end and no beginning. He offers both of them, and thus makes both a beginning and an end.