Before Maricha and Ravana arrived in Panchavati, Sri Rama had Sita seek shelter with Agni and replaced her with an illusionary Sita
Lord Shiva said—'Oh Parvati! Here, becoming aware of all the conspiracies of Ravana, Sri Ram advised Sita when they were alone, 'Oh Sita! Listen to what I have to tell you. Oh the auspicious one! Ravana will come to you disguised as a mendicant. Hence, by my orders, you must enter the holy fire leaving behind your lookalike shadow in this hut, and stay there in the fire in an invisible form for one year. After that, after the killing of Ravana, you shall get back to me once again.’
Hearing these words of Sri Ram, Sita did as she was told. She became invisible in fire, leaving behind a shadowy Sita. Then that Maya-Sita saw that Maya-deer, came to Sri Ram and smilingly, politely said to him, 'Oh Sri Ram! Look at this golden deer which is adorned by gems and jewels. Ah! What marvellous spots are there on his body and how fearlessly it is wandering and frolicking here! Oh Lord! Tie it and bring it to me; let it by my playmate'.
- Canto 7, Aranya Kanda, Adhyatma Ramayana.
It was Maya Sita who was abducted by Ravana as per the text.
After Ravana had been slain, Maya Sita disappeared during the Agni Pariksha and was the real Sita once again appeared
Oh Hari (Sri Ram)! You had created an illusionary/shadowy Sita for the express purpose of destroying (i.e., killing) Ravana along with his sons, kiths and kins and comrades etc. Oh Lord! By doing so, you have removed the burden of this earth.The illusionary/shadowy Sita who was like an image of her real self has vanished after having accomplished the task for which she was created.’ Hearing these words of the fire God, Sri Ram worshipped and honoured him with great delight and a joyful heart and then he gladly accepted Janki (Sita) who was most happy and cheerful (at this happy turn of events).
- Canto 13, Lanka kanda, Adhyatma Ramayana.
Many Puranas like Skanda, Kurma, Devi Bhagavata and Brahma vaivarta also mentions this story.