Targum zu Bamidbar 29:13

and offer a sacrifice, an oblation to be received with favour before the Lord: thirteen young bullocks proceeding daily and diminishing their number, (in all) seventy for the seventy nations, and offering them by thirteen orders; two rams, which you shall offer by two orders; lambs of the year fourteen, unblemished, to be offered by eight orders, offering six of them, by two and two, and two of them one by one, they shall be perfect.