CC Madhya 9.211-212
tāṁ jahāra daśa-grīvaḥ
sītā vahni-puraṁ gatā
chāyā-sītā viveśa sā
vahniḥ sītāṁ samānīya
sītayā — by mother Sītā; ārādhitaḥ — being called for; vahniḥ — the fire-god; chāyā-sītām — the illusory form of mother Sītā; ajījanat — created; tām — her; jahāra — kidnapped; daśa-grīvaḥ — the ten-faced Rāvaṇa; sītā — mother Sītā; vahni-puram — to the abode of the fire-god; gatā — departed; parīkṣā-samaye — at the time of testing; vahnim — the fire; chāyā-sītā — the illusory form of Sītā; viveśa — entered; sā — she; vahniḥ — the fire-god; sītām — the original mother Sītā; samānīya — bringing back; tat-purastāt — in His presence; anīnayat — brought back.
“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.