Bhagavad Gita 4.7
यदा यदा हि धर्मस्य ग्लानिर्भवति भारत। अभ्युत्थानमधर्मस्य तदात्मानं सृजाम्यहम्।।4.7।।
yadā yadā hi dharmasya glānirbhavati bhārata| abhyutthānamadharmasya tadātmānaṃ sṛjāmyaham||4.7||
yada yada hi dharmasya glanirbhavati bharata abhyutthanamadharmasya tadatmanam srijamyaham4.7
Meaning
Whenever righteousness declines and unrighteousness rises, Krishna manifests Himself. This is the divine promise of God's intervention in the affairs of the world to restore balance.
हे भारत! जब-जब धर्म की हानि और अधर्म की वृद्धि होती है, तब-तब मैं स्वयं को प्रकट करता हूँ।
This is verse 7 of 42 in Chapter 4 (Jnana Karma Sanyasa Yoga) of the Bhagavad Gita.