https://www.npr.org/2019/05/29/727889232/read-special-counsel-robert-muellers-full-statement
-----------
Now, I hope and expect this to be the only time that I will speak to you in this manner.
...

And the report is my testimony. I would not provide information beyond that which is already public in any appearance before Congress.
...

So, beyond what I've said here today and what is contained in our written work, I do not believe it is appropriate for me to speak further about the investigation or to comment on the actions of the Justice Department or Congress.
------------

he said in no uncertain terms that he hopes/expects not to ever speak about the investigation again. those are his intentions.

but he doesnt get to decide that. he has to testify when legally required to do so. so he said if that if that happens...then it would basically just be a rehashing of the written report (aka a waste of time).

he doesnt wanna testify to congress. period. he even called it 'inappropriate'.

an investigator that expects not to come into congress and speak about their *completed* investigation is an inexplicable position. there are oversight committees in congress that exist specifically for that exact function and that have been requiring that testimony since forever. he should know. hes done it over 50 times. theres no logical reason for him to be acting brand new.