One of my German readers asked whether it is true that the forefathers kept the Torah before the official Matan Torah at Mount Sinai.
Yes, indeed, there is such a concept in Judaism and I also learned this in Yeshiva: That the Jewish forefather already kept various Torah Mitzvot before G-d gave the Torah to the Jews at Mount Sinai. The Gemara (rabbinic discussions) in Talmud Yoma 28b states that Avraham kept the Mitzvah of various Eruvin.
The Torah in Bereshit / Genesis 18:8 teaches that Avraham kept the dietary laws, as he first placed dairy items in front of the so-called Angels appearing at his tent. Only afterwards, Avraham served the full meal consisting of meat. From here we learn that Avraham kept dairy and meat separate.
The very first Mitzvah listed in the Torah is PRU u’REVU (Be fruitful and Multiply), Bereshit / Genesis 1:28. G-d wanted mankind to procreate and populate the entire planet. A male Jew has the obligation to procreate whereas Jewish women and all Noachides are exempt from this Mitzvah. There is an opinion that woman don’t necessarily need to procreate because giving birth could be harmful to a female body.
The second Mitzvah listed in the Torah is Brit Milah (Circumcision). See Bereshit / Genesis 17:10.
There are different rabbinic opinions on whether the forefathers really kept the Torah before Mount Sinai. Some Rabbis say YES whereas other express their doubts. However, various Torah laws were given to the Jews before the official Matan Torah.