It is mentioned in Valmiki Ramayana, Uttara kanda, Sarga 17 , When Rishi Augusta (अगस्त्य) told Lord Ram about the Vedavati (वेदवती) and her next birth as Mata Sita (सीता).
Vedavati (वेदवती) is the daughter of Brahmarishi Kushadhvaja (कुशध्वज), who is the son of Brihaspati (बृहस्पति), Guru of the Devatas.
Vedavati and her desire to marry Lord Vishnu
Her father wants his child to have Lord Vishnu as her husband. He thus rejects many powerful kings and celestial beings who sought his daughter’s hand. Outraged by his rejection, King Sambhu murders her father during his sleep.
Vedavati continues to live in the ashram of her parents, meditating night and day and performing a great tapasya to win Vishnu as her husband.
Ravana and his assault towards Vedavati
Ravana, the emperor of Lanka once found Vedavati sitting in ashram and is attracted towards her incredible beauty. He proposed her and was rejected. Ravana mocked her austerities and her devotion to Vishnu. Ravana on finding himself rejected and unwelcomed, he grabbed Vedavati’s hair and assaulted her.
This made Vedavati furious and she cut off her hair, and said to the Ravana
“Since you have insulted me , therefore I will be born again for your death. If I have performed dharma, charity and yajna, than I should be born as Ayonija – अयोनिजा ( not born from a womb) and should be a daughter to a dharmic father”
Vedavati entered into the burning fire.
Rebirth of Vedavati
Thereafter in the second life Vedavati is born from a lotus, Ravana came to know about this and took the girl with him. Ravana took the girl to his palace and showed it to his minister. The minister after looking the girl said to the Ravana :-
“Dear King ! this girl has qualities that can cause harm to the well being of Lanka “
On the advise of minister Ravana throw the girl in the ocean, from the ocean the girl was reached to the land, from there to the fields of King Janaka, where he was trying to setup a Yajna pavilion.
Vedavati was born in Satyuga and than Mata Sita was born in Treta Yuga.