That isn't his quote. But this is (https://www.vox.com/2019/5/29/18644249/robert-mueller-statement-trump-special-counsel-obstruction) :
“If we had had confidence that the president clearly did not commit a crime, we would have said so,” Mueller said Wednesday. “We did not, however, make a determination as to whether the president did commit a crime.”
It’s a striking statement — and a reiteration of what he wrote in the report the Justice Department released nearly in full last month. For weeks, Republican lawmakers and Barr have repeatedly said the report showed no collusion and no obstruction. Mueller is making a point to say that’s not necessarily the case and that he can’t make the call — because of Department of Justice policy.
Here’s Mueller in his own words:
It explains that under long-standing department policy a president cannot be charged with a federal crime while he is in office. That is unconstitutional. Even if the charge is kept under seal and hidden from public view, that too is prohibited. The special counsel’s office is part of the Department of Justice and by regulation it was bound by that department policy. Charging the president with a crime was, therefore, not an option we could consider.
[…]
It would be unfair to potentially accuse somebody of a crime when there can be no court resolution of the actual charge. So that was Justice Department policy. Those were the principles under which we operated and from them we concluded that we would not reach a determination one way or the other about whether the president committed a crime.
