According to four different people who are singing like songbirds to multiple news outlets. The New York Times reports that last June, Trump, tried to apparently fire Mueller over “alleged conflicts of interests”  stemming from Mueller’s prior knowledge of Jared Kushner’s legal affairs.
Even worse, The Washington Post, reports that Trump was persuaded out of his decision to fire Mueller after aides desperately spent “hours trying to convince him that if he does, he, would almost certainly be removed from the White House”.
The White House has repeatedly denied the report since publication, of course, citing that it is fake news.