Found it.
Office of Legal Counsel
“The OLC memorandum concluded that all federal civil officers except the President are subject to indictment and criminal prosecution while still in office; the President is uniquely immune from such process.
... we agree with the 1973 conclusions that indicting and prosecuting a sitting President would “prevent the executive from accomplishing its constitutional functions” and that this impact cannot “ be justified by an overriding need” to promote countervailing and legitimate government objectives.”
https://www.justice.gov/sites/default/files/olc/opinions/2000/10/31/op-olc-v024-p0222.pdf
That does not preclude Congress from determining what a President can or cannot be Impeached for.
True. Mueller can only present his findings in regards to the President. His recommendation is meaningless and irrelevant, just like Ken Starr in the Clinton Investigation. However, I believe that the House can impeach for any reason they so deem (but very hard to do), and the Senate only can convict with 2/3 majority. I believe that the timing is a shot across the bow for Sessions while he considers firing McCabe. Also, the IG is getting ready to release his report which will set the course for all of the investigations.