Shatapatha Brahmana 7.1.1.33

7.1.1.33

atha lokampṛṇāmupadadhāti | tasyā upari bandhustisraḥ pūrvāstrivṛdagniryāvānagniryāvatyasya mātrā tāvantamevainametaccinoti daśottarāstāsāmupari bandhurdve vāgre 'tha daśāthaikāmevaṃ hi citiṃ cinvanti tāstrayodaśa sampadyante tasyokto bandhuḥ

He then puts on a space-filling one: the significance of that one (will be explained) further on. Three there are in front,--threefold is Agni: as great as Agni is, as great as is his measure, so great he thus builds him;--and ten those that follow,--the significance of these (will be explained) further on. Or first two, then ten, and then one, for in this way they build up the pile,--these amount to thirteen: the significance of this has been told.