Origin of Three Weeks/Nine Days customs?
When discussing the period of mourning leading up to Tisha B'av, the Mishnah (Taanit 4:7) only discusses the "week in which Tisha B'av falls." This is codified even as recently as the Shulchan Aruch (Orach Chaim 551:3), with the Rema ad. loc. commenting there that "we have the custom to be stringent on all this from Rosh Chodesh until after the fast." Throughout the following sections, Rav Yosef Karo only discusses the laws associated with the Nine Days in conjunction with just the week in which Tisha B'av falls, the Rema occassionally noting on certain points that Ashkenazim keep those laws from 17 Tammuz (ex. Orach Chaim 551:4).
What is the earliest source which records extending some of these practices back to the beginning of Av or to the fast of Tammuz?