Srimad-Bhagavatam: Canto 12 - Chapter 1 - Verse 31
Sanskrit:
यदा देवर्षय: सप्त मघासु विचरन्ति हि ।तदा प्रवृत्तस्तु कलिर्द्वादशाब्दशतात्मक: ॥ ३१ ॥
ITRANS:
yadā devarṣayaḥ saptamaghāsu vicaranti hitadā pravṛttas tu kalirdvādaśābda-śatātmakaḥ
Translation:
When the constellation of the seven sages is passing through the lunar mansion Maghā, the Age of Kali begins. It comprises twelve hundred years of the demigods.