Where is that hinted? I don't remember that ever being mentioned.
It wasn't in any of the games and it was also never hinted that it was really Terra's heart, it was only a guess by an interviewer in the Birth by Sleep-Ultimania and Nomura refused to give any definite information.
Nomura in BBS Ultimania said:-- In "KHI", the robed man, Ansem the Wise's apprentice Xehanort's Heartless (the body of Xehanort's Heartless before he inhabited Riku's body), appeared on the Destiny Islands. Is this because it was Master Xehanort's homeworld?
Nomura: That may be one reason, but if he somehow had some of Terra's memories, we can consider that it might be because he had laid eyes on Riku previously. In terms of the elements of Terra and Master Xehanort, I think the questions of how their power is related and divided, as well as who has whose memories and heart, will become a key to the story * after this one.
-- Is that to say that there's a possibility that within Xehanort's Heartless, the darkness that sleeps in Riku's heart, there might remain something of Terra...?
Nomura: That's also a riddle connected to the next story,* so everyone please use your imagination.
So in short, it's only a theory, nothing substantial where you can put your finger on.