CC Adi-lila 17.164 - Brahma-vaivarta Purana
Sri Caitanya-caritamrta
CC Adi-lila
CC Adi 17.164, Translation and Purport:
“"In this Age of Kali, five acts are forbidden: the offering of a horse in sacrifice, the offering of a cow in sacrifice, the acceptance of the order of sannyāsa, the offering of oblations of flesh to the forefathers, and a man"s begetting children in his brother's wife.’
This is a quotation from the Brahma-vaivarta Purāṇa (Kṛṣṇa-janma-khaṇḍa 185.180).