Bhagavad Gita 4.7
Whenever spirituality decays and materialism is rampant, then, O Arjuna, I reincarnate myself.
यदा यदा हि धर्मस्य ग्लानिर्भवति भारत।
अभ्युत्थानमधर्मस्य तदाऽऽत्मानं सृजाम्यहम्॥yadā yadā hi dharmasya glānirbhavati bhārata|
abhyutthānamadharmasya tadā''tmānaṃ sṛjāmyaham||
English translation (above): Shri Purohit Swami (1935), public domain. Sanskrit text via github.com/gita/gita (Unlicense).