Yada Yada Hi Dharmasyaयदा यदा हि धर्मस्य

Bhagavad Gita 4.7 teaches a devotional principle from Krishna’s dialogue with Arjuna.

Bhagavad Gita 4.7
यदा यदा हि धर्मस्य ग्लानिर्भवति भारत । अभ्युत्थानमधर्मस्य तदात्मानं सृजाम्यहम् ॥
भावार्थ
जब-जब धर्म का क्षय होता है और अधर्म बढ़ता है, तब-तब मैं स्वयं प्रकट होता हूँ।
Meaning
Whenever dharma declines and adharma rises, I manifest myself, descendant of Bharata.

Source: Bhagavad Gita 4 Sanskrit checked against SanskritDocuments: https://www.sanskritdocuments.org/doc_giitaa/bhagvadnew.html ; meanings checked against IIT Kanpur Gita Supersite and Vedabase chapter 4: https://www.gitasupersite.iitk.ac.in/srimad?field_chapter_value=4&field_nsutra_value=7&language=dv ; https://vedabase.io/en/library/bg/4/