I heard it for the first time from a purohit around me but I am confused because I have never heard such a story.
So, I would like to know what is the story and why did Ravana abduct lord Rama's mother Kausalya?
In my knowledge, this story is not in Valmiki Ramayana or Ramcharitmanas.
Though, I found this in ANANDA RAMAYANAM: SAra KAndam: Chapter 1.
Below is the answer for Why:
Ravana calls for Brahma and asks Him who will cause his (Ravana's) death. Brahma tells him that Lord Narayana will be born to Dasaratha and Kausalya as Rama and will kill Ravana. He also tells him that Dasaratha and Kausalya are going to be married in 5 days.
Then Ravana went to Ayodhya and attacked Dasaratha who was in a boat. He thought that he has killed Dasaratha but actually Dasaratha escaped with Sumanth in a peice of boat.
Then, Ravana abducted Kauslya and put in a box. While returning to Lanka, he handed over the box to shark. But another shark attacked that shark and it put the box nearby. Meanwhile, Dasaratha and Sumanth also comes to that place and opens the box and entered in it. The shark again took the box.
Ravana calls Brahma and boasts that he has killed Dasaratha and abducted Kausalya. Brahma laughs and tells him that they have already been married. The angered Ravana calls for the shark and opens the box; out come the three people. The scared Ravana tries to kill them with his sword. Brahma tries to console him saying, " You put only one girl in the box and now she has become three. Is it not possible that many more will be coming out of the box? Rama could be one of them. Just close the box and send it to Ayodhya".
Hindi translation by Ramtej Pandey and Sanskrit verses for Adhyatam Ramayana can be found on Archive.