Shatapatha Brahmana 8.7.1.8

8.7.1.8

yadvevaite atropadadhāti | prajāpatireṣo 'gniḥ saṃvatsara u prajāpatistasya śira eva pañcamī citiḥ śiro 'sya śiśira ṛtustadyadete atropadadhāti yadevāsyaite ātmanastadasminnetatpratidadhāti tasmādete atropadadhāti

And, again, as to why he now lays down these two;--this Agni (the fire-altar) is Pragâpati (the lord of generation), and Pragâpati is the year: the fifth layer is his (Agni's) head, and the dewy season is its (the year's) head; and when he now lays down these two (bricks), he thereby restores to his (or its) body what these two are thereto: this is why he now lays down these two (bricks).