Burdick v. United States (1915):

There are substantial differences between legislative immunity and a pardon; the latter carries an imputation of guilt and acceptance of a confession of it, while the former is noncommittal, and tantamount to silence of the witness.

But it was dicta so still debatable if precedent.

Burdick v. United States (1915):

There are substantial differences between legislative immunity and a pardon; the latter carries an imputation of guilt and acceptance of a confession of it, while the former is noncommittal, and tantamount to silence of the witness.