Are there places in halacha where content from a purely midrashic source (like midrash rabbah, pesikta, sifra, sifrei, etc.) find their way into halacha? Anything in the Talmud is excluded for the purposes of this question. I'm curious if something went from a midrash to codification in halacha (perhaps via a rishon), without appearing in the Talmud as well.
A Yissachar-Zevulon relationship is one in which the Yissachar learns Torah all day and the Zevulun works all day and they split their respective profits in this world and the next world. The halachos of this partnership are discussed in YD 246:1, Minchas Yitzchak 6:100 and 7:87, Igros Moshe YD 4:37, Avkas Rocheil 2, Yabia Omer 7 YD 17. The source for this relationship is recorded in several Midrashim - Bereishis Rabbah 72:5, 99:9, Vayikra Rabbah 25:2, Agadas Bereishis 82, Yalkut Mishlei 934 - but AFAICT it appears nowhere in Gemara. (The Midrash does darshen two Mishnayos, Zevachim 1:2 and Tohoros 8:7, as referring to this practice with how it records a Tanna's name, but those sugyos never discuss it explicitly.)
Doing dangerous things during the Nine Days. I will refer readers to a question of mine on the topic for more detailed sources in Halacha. Eichah Rabbah 1:29 is ultimately the source for this prohibition; Pesachim 111b only made its way into that post in the context of discussing Ketev Meriri.
While saying Baruch Shem during Shema is explained by Pesachim 56a (Yaakov said it in response to his children saying Shema Yisrael) and Devarim Rabbah 2:35 (Moshe taught it from the angels’ song), only in the latter is it explained why we say the verse aloud on Yom Kippur (since we are like angels on that day, we are able to say the praise of the angels).
The Rema (YD 240:24) quotes a Midrash (Bereishis Rabbah 94:5) as a source to honor one’s grandparents. I address this at length here.
That a father makes a Baruch Shepetarani when his son reaches Bar Mitzvah (Rema, OC 225:2) is based on Bereishis Rabbah 63:10 and Yalkut Shimoni on the Torah 110:28. The Mishnah Berurah (ad. loc. §8) explains that it’s for this reason - that the Beracha is not found in the Gemara - that one does not make the Beracha with Hashem’s Name.