CC Madhya-lila 18.008 - Padma Purana
Sri Caitanya-caritamrta
CC Madhya-lila
CC Madhya 18.8, Translation and Purport:
“"Just as Śrīmatī Rādhārāṇī is most dear to Lord Kṛṣṇa, so Her lake, known as Rādhā-kuṇḍa, is also very dear to Him. Of all the gopīs, Śrīmatī Rādhārāṇī is certainly the most beloved."
This is a verse from the Padma Purāṇa.