Trump first made the claim in a phone interview with Fox News:

His father was with Lee Harvey Oswald prior to Oswald’s being – you know, shot. I mean, the whole thing is ridiculous.

Trump defended his remark, saying, “All I did is point out the fact that on the cover of the National Enquirer there was a picture of him and crazy Lee Harvey Oswald having breakfast…This was a magazine that, in many respects, should be well respected. I mean if that was The New York Times, they would have gotten Pulitzer Prizes for their reporting.”

Trump also attacked Cruz’s wife’s appearance on Twitter.

BBC News – Trump links Cruz’s father to Kennedy assassination
Politico – Trump revives rumor linking Cruz’s father to JFK assassination