The source is the Ran on the Rif in the first perek of K'subos (and it's quoted in the Taz EH 34), that "Rabbeinu Tam z"l higiah," which indicates it was Rabbeinu Tam's change.
EMT»
Yishar Koch'cha
One Talmid of mine once even complained that RT didn't go far enough to remove the ambiguity!
Shalom and Regards, RRW