Śatapatha Brāhmaṇa ("of a hundred paths") 7.1.1.35

7.1.1.35

ekaviṃśatirveva pariśritaḥ | dvādaśa māsāḥ pañca 'rtavastraya ime lokā ayamagniramuto 'dhyekaviṃśa imaṃ tadagnimamuṣminnāditye pratiṣṭhāpayati tadyadetā evamupadadhātyetāvevaitadanyo 'nyasminpratiṣṭhāpayati tāvetāvanyo 'nyasminpratiṣṭhitau tau vā etāvatra dvāvekaviṃśau sampādayatyatra hyevemau tadobhau bhavata āhavanīyaśca gārhapatyaśca

Moreover, there are twenty-one enclosing-stones,--twelve months, five seasons, these three worlds, and that Agni from yonder (sun) as the twenty-first: this Agni he thus establishes in yonder sun. And inasmuch as he puts on those (bricks) in this way, he thereby establishes those two (the sun and the fire) in each other, and (accordingly) those two are established in each other; for both of them he now makes out to be the twenty-first, and both of them are then here, as the Âhavanîya and the Gârhapatya.