Manusmriti
यदेतत् परिसङ्ख्यातमादावेव चतुर्युगम् । एतद् द्वादशसाहस्रं देवानां युगमुच्यते ॥ ७१ ॥
These twelve thousand (years) which thus have been just mentioned as the total of four (human) ages, are called one age of the gods.
english translation
yadetat parisaGkhyAtamAdAveva caturyugam । etad dvAdazasAhasraM devAnAM yugamucyate ॥ 71 ॥
hk transliteration by Sanscript