CC Madhya 1.117

tāhāñi karila kūrma-purāṇa śravaṇa
māyā-sītā nileka rāvaṇa, tāhāte likhana
Word for word: 
tāhāñi — there; karila — did; kūrma-purāṇa — of the Kūrma Purāṇa; śravaṇa — hearing; māyā-sītā — a false Sītā; nileka — kidnapped; rāvaṇa — by Rāvaṇa; tāhāte — in that book; likhana — it is stated.
Translation: 
At Rāmeśvara, Śrī Caitanya Mahāprabhu had a chance to read the Kūrma Purāṇa, in which He discovered that the form of Sītā kidnapped by Rāvaṇa was not that of the real Sītā but a mere shadow representation.
Purport: 

The Kūrma Purāṇa states that this shadowy Sītā was placed into a fire as a test of chastity. It was Māyā-sītā who entered the fire and the real Sītā who came out of the fire.