When the Supreme Lord has appeared on earth as Kalki, the maintainer of religion, Satya-yuga will begin, and human society will bring forth progeny in the mode of goodness.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
यदावतीर्णो भगवान् कल्किर्धर्मपतिर्हरिः । कृतं भविष्यति तदा प्रजा सूतिश्च सात्त्विकी
yadāvatīrṇo bhagavān kalkirdharmapatirhariḥ | kṛtaṃ bhaviṣyati tadā prajā sūtiśca sāttvikī
SB 12.2.23