This is a verse spoken by Nārada Muni in the Lalita-mādhava-nāṭaka (6.14), a drama written by Śrīla Rūpa Gosvāmī.
The wonderful feelings of the gopis in ecstatic parama-rasa with Krsna constitute the greatest mystery in spiritual life - CC Madhya 9.150
Expressions researched:
"The wonderful feelings of the gopīs in ecstatic parama-rasa with Kṛṣṇa constitute the greatest mystery in spiritual life"
Sri Caitanya-caritamrta
CC Madhya-lila
“‘Once Lord Śrī Kṛṣṇa playfully manifested Himself as Nārāyaṇa, with four victorious hands and a very beautiful form. When the gopīs saw this exalted form, however, their ecstatic feelings were crippled. A learned scholar, therefore, cannot understand the gopīs’ ecstatic feelings, which are firmly fixed upon the original form of Lord Kṛṣṇa as the son of Nanda Mahārāja. The wonderful feelings of the gopīs in ecstatic parama-rasa with Kṛṣṇa constitute the greatest mystery in spiritual life.’”.
“‘Once Lord Śrī Kṛṣṇa playfully manifested Himself as Nārāyaṇa, with four victorious hands and a very beautiful form. When the gopīs saw this exalted form, however, their ecstatic feelings were crippled. A learned scholar, therefore, cannot understand the gopīs’ ecstatic feelings, which are firmly fixed upon the original form of Lord Kṛṣṇa as the son of Nanda Mahārāja. The wonderful feelings of the gopīs in ecstatic parama-rasa with Kṛṣṇa constitute the greatest mystery in spiritual life.’”