Srimad-Bhagavatam: Canto 12 - Chapter 1 - Verse 28
Sanskrit:
यदा कर्मसु काम्येषु भक्तिर्यशसि देहिनाम् ।तदा त्रेता रजोवृत्तिरिति जानीहि बुद्धिमन् ॥ २८ ॥
ITRANS:
yadā karmasu kāmyeṣubhaktir yaśasi dehināmtadā tretā rajo-vṛttiriti jānīhi buddhiman
Translation:
O most intelligent one, when the conditioned souls are devoted to their duties but have ulterior motives and seek personal prestige, you should understand such a situation to be the age of Tretā, in which the functions of passion are prominent.