Most people are only familiar with Dasharatha's four sons - Rama, Lakshmana, Bharata, and Shatrughna. But there is a common story that Dasharatha also had a daughter. The story as I understand it goes roughly as follows: before the birth of Rama or any of Dasharatha's other sons, Dasharatha and Kausalya had a daughter by the name of Shanta. Now Kausalya's sister Varshini was the wife of Romapada king of Anga, and the couple were unable to have children. So Varshini once asked Dasharatha, jokingly, whether he would give his daughter Shanta to her and her husband. But Dasharatha took the request seriously and agreed to give his daughter to Romapada. Romapada raised Shanta, and eventually married her off to Rishyasringa, the sage who carried out Dasharatha's Putrakameshti Yagna that produced Rama, Lakshmana, Bharata and Shatrughna.
But none of this is mentioned in the Ramayana. The (very entertaining) story of Rishyasringa and how he got married to Shanta is described in great detail, but Shanta is just described as Romapada's daughter; for instance it says "the king of Anga will beget a fortunate girl named Shanta" and when Dasharatha talks to Rompada he just says "your daughter princess Shanta". There's no mention that Shanta is actually the biological daughter of Dasharatha. The Vana Parva of the Mahabharata similarly says "And [R]omapada, the desire of his heart fulfilled, bestowed his daughter Santa on Rishyasringa in marriage."
So my question is, what scriptures, if any, mention the fact that Shanta is Dasharatha's daughter? There are many retellings of the Ramayana in the Puranas, so maybe one of them mentions it? Or is it a later folk legend with no basis in Hindu scripture?