Shatapatha Brahmana 10.1.4.8

10.1.4.8

tā vā etāḥ ṣaḍiṣṭakācitayaḥ ṣaṭ purīṣacitayastaddvādaśa dvādaśa māsāḥ saṃvatsaraḥ saṃvatsaro'gniryāvānagniryāvatyasya mātrā tāvataiva tatprajāpatirekadhājaramamṛtamātmānamakuruta tathaivaitadyajamāna ekadhājaramamṛtamātmānaṃ kurute

These, then, are six layers of bricks, and six layers of earth, that makes twelve,--the year (consists of) twelve months, and Agni is the year: as great as Agni is, as great as is his measure, with so much did Pragâpati then make his body uniformly undecaying and immortal; and in like manner does the Sacrificer now make his body uniformly undecaying and immortal.