I believe the requirement to have single-member districts was introduced with 2 U.S.C. § 2c in 1967. The name for the previous practice was "at Large" election

I believe the requirement to have single-member districts was introduced with 2 U.S.C. § 2c in 1967. The name for the previous practice was "at Large" election, and the text of the law[0] includes a reference to that practice as part of a transitional measure: "a State which is entitled to more than one Representative and which has in all previous elections elected its Representatives at Large may elect its Representatives at Large to the Ninety-first Congress" https://www.law.cornell.edu/uscode/text/2/2c https://www.law.cornell.edu/uscode/text/2/2c