Shatapatha Brahmana 6.8.2.7

6.8.2.7

caturbhirapādatte | tadye catuṣpādāḥ paśavastairevainametatsambharatyatho annaṃ vai paśavo 'nnenaivainametatsambharati tribhirabhyavaharati tatsapta saptacitiko 'gniḥ sapta 'rtavaḥ saṃvatsaraḥ saṃvatsaro 'gniryāvānagniryāvatyasya mātrā tāvattadbhavati

With four (verses) he takes (some of the ashes);--he thereby supplies him (Agni) with four-footed animals; and animals being food, it is with food he thus supplies him. With three (verses) he takes (the ashes) down (to the water),--that makes seven, for of seven layers consists the fire-altar, seven seasons are a year, and the year is Agni: as great as Agni is, as great as is his measure, so great does this become.