The Lord says in Bhagavad-gita (BG 4.7) - Whenever and wherever there is a decline in religious practice, O descendant of Bharata, and a predominant rise of irreligion - at that time I descend Myself.

Expressions researched:
"The Lord says in Bhagavad-gītā (BG 4.7): yadā yadā hi dharmasya glānir bhavati bhārata abhyutthānam adharmasya tadātmānaṁ sṛjāmy aham" |"Whenever and wherever there is a decline in religious practice, O descendant of Bharata, and a predominant rise of irreligion—at that time I descend Myself"

Srimad-Bhagavatam

SB Canto 10.1 to 10.13

"Whenever and wherever there is a decline in religious practice, O descendant of Bharata, and a predominant rise of irreligion—at that time I descend Myself." Whenever Kṛṣṇa comes, once in a day of Brahmā, He comes to the house of Nanda Mahārāja in Vṛndāvana. Kṛṣṇa is the master of all creation (sarva-loka-maheśvaram (BG 5.29)).

SB 10.5.13, Translation and Purport

Now that the all-pervading, unlimited Lord Kṛṣṇa, the master of the cosmic manifestation, had arrived within the estate of Mahārāja Nanda, various types of musical instruments resounded to celebrate the great festival.

The Lord says in Bhagavad-gītā (BG 4.7):

yadā yadā hi dharmasya

glānir bhavati bhārata

abhyutthānam adharmasya

tadātmānaṁ sṛjāmy aham

"Whenever and wherever there is a decline in religious practice, O descendant of Bharata, and a predominant rise of irreligion—at that time I descend Myself." Whenever Kṛṣṇa comes, once in a day of Brahmā, He comes to the house of Nanda Mahārāja in Vṛndāvana. Kṛṣṇa is the master of all creation (sarva-loka-maheśvaram (BG 5.29)). Therefore, not only in the neighborhood of Nanda Mahārāja's estate, but all over the universe—and in all the other universes—musical sounds celebrated the auspicious arrival of the Lord.