עֵקֶב, אֲשֶׁר-שָׁמַע אַבְרָהָם בְּקֹלִי; וַיִּשְׁמֹר, מִשְׁמַרְתִּי, מִצְוֹתַי, חֻקּוֹתַי וְתוֹרֹתָי
...because Abraham obeyed Me and kept My charge, My commandments, My statutes and My laws.
This is the famous posuk that causes legions of yeshiva boys to imagine the Patriarchs assembling eruvim and slaughtering goats in honor of the Passover. Usually, they attribute the idea that Avraham and his family followed every jot and tittle of the not-yet-revealed divine law to Rashi, but in fact its found first in the Talmud, tractate Yoma, where an odd aggadah announces that they kept an observance (eruv tavshilim) designed to solve a problem caused by a holiday that hadn't been invented yet.
On the spot, Rashi cites the Gemarah, not to praise the Patriarchs, but because he's spoted an anamoly. The verse says Toratai. Two torahs? Ah, answers Rashi: We mean the writtten and the oral. Both were kept by our ancient forefathers.
Rashbam, pace his brilliant grandfather, does not agree. He says, that the ikar pshuto (his words) is that the word "Torasai" means only the 'obvious mitzvot' like stealing, adultery, coveting, justice, and welcoming guests. These rules were in affect before Matan Torah, (paging Plato) (or see here) but were renewed and expounded at Sinai. In short, Rashbam is not troubled by the use of a plural (Torasai) and unlike Rashi, has no reason to rely on the midrash for pshat. According to him the Avot kept the "obvious laws" and nothing else.
Chizkuni, Ibn Ezra and Radak agree with Rashbam in general, but not on the particulars. All three agree with Rashbam, and reject the Midrash, along with the idea that every divine law was known to the Patriarchs, and observed by them. However, they each give a different defintion to the words מִשְׁמַרְתִּי, מִצְוֹתַי, חֻקּוֹתַי וְתוֹרֹתָי. For instance, Chizkuni says "Torah" means Avraham's original trip to Israel (based on a use of the word elsewhere) whereas the Ibn Ezra says "Torah" refers to the mitzvah od brit mila.
(There's a long post I could write about how the Chizkuni, Ibn Ezra, Radak and the Rashbam disagree on the meaning of מִשְׁמַרְתִּי, מִצְוֹתַי, חֻקּוֹתַי וְתוֹרֹתָי in a way that reveals their individual approaches to pshat, but naah: that's why God gave us Josh Waxman)
Meanwhile, the Ramban pays homage to Rashi and the midrash, and finds a way to reconcile it with the plain fact that the Avot did NOT follow the commandments (Yaakov's sister-wives being just one example) but, in the end, concludes that Torasai means only the actual mitzvoth he was given: e.g. Brit Milah & Noachide laws. Nothing more.