Srimad-Bhagavatam: Canto 12 - Chapter 1 - Verse 39
Sanskrit:
कृतं त्रेता द्वापरं च कलिश्चेति चतुर्युगम् ।अनेन क्रमयोगेन भुवि प्राणिषु वर्तते ॥ ३९ ॥
ITRANS:
kṛtaṁ tretā dvāparaṁ cakaliś ceti catur-yugamanena krama-yogenabhuvi prāṇiṣu vartate
Translation:
The cycle of four ages — Satya, Tretā, Dvāpara and Kali — continues perpetually among living beings on this earth, repeating the same general sequence of events.