Andres v. United States
In Andres v. United States, 333 U.S. 740, 748, 92 L. Ed. 1055, 68 S. Ct. 880 (1948) the court held that, when a jury decision is required in federal court, the jury's decision must be unanimous: "In criminal cases this requirement of unanimity extends to all issues -- character or degree of the crime, guilt and punishment -- which are left to the jury." See Fed. R. Crim. P. 32(c)(1).
In Andres, the defendant was tried for first degree murder.
Under the applicable statute, this crime was punishable by death unless the jury qualified its verdict by adding the phrase "without capital punishment" -- in which case the sentence was life.
The jury was instructed that their verdict must be unanimous, and in response to a question about the qualified verdict, the trial judge issued special instructions, providing in relevant part as follows:
And, finally, you will recall I said that you are instructed that before you may return a qualified verdict of murder in the first degree without capital punishment, that your decision to do so must, like your regular verdict, be unanimous.