James Callender floated the story in 1802 after Jefferson jilted him for a job. The Federalist press ran with it, but Jefferson never commented on it publicly and he and his family denied it privately. The story went away by the time Jefferson died and it was only revitalized in the late 19th century. I talk about this in my PIG to the Founding Fathers. I am in the camp that does not think it was Jefferson, but his brother, Randolph, that fathered Hemings children. William G. Hyland’s book on the topic, In Defense of Thomas Jefferson, is pretty good.