CC Madhya-lila 09.211-212 - Kurma Purana
Sri Caitanya-caritamrta
CC Madhya-lila
CC Madhya 9.211-212, Translation and Purport:
"When he was petitioned by mother Sītā, the fire-god, Agni, brought forth an illusory form of Sītā, and Rāvaṇa, who had ten heads, kidnapped the false Sītā. The original Sītā then went to the abode of the fire-god. When Lord Rāmacandra tested the body of Sītā, it was the false, illusory Sītā that entered the fire. At that time the fire-god brought the original Sītā from his abode and delivered her to Lord Rāmacandra."
These two verses are taken from the Kūrma Purāṇa.