Shatapatha Brahmana 10.1.2.5

10.1.2.5

śira evāgniḥ prāṇo mahāvratamātmā mahadukthaṃ tasmādetāni sarvāṇi sahopeyātsaha hi śiraḥ prāṇa ātmā tadyadagniḥ prathamaścīyate śiro hi prathamaṃ jāyamānasya jāyate tasmādyatraitāni sarvāṇi saha kriyante mahadevokthamātamāṃ khyāyata ātmā hi mahaduktham

The Fire-altar, indeed, is the head, the Great Rite the breath, and the Great Litany the body: one ought therefore to undertake all these together, for head, breath, and body belong together; and as to why the Fire-altar is built first, it is because of him who is born the head is born first; and hence, whenever all these are undertaken together the Great Litany, indeed, is accounted the highest (âtamâm), for the Great Litany is the body (or self, âtman).