http://revach.net/article.php?id=2217

Section: Parshas HaShavua   Category: Quick Vort
Pashas Behar: Chasam Sofer Shmita Teaches Us That Torah Comes From Sinai
The parsha starts by telling us that Hashem told Moshe on Har Sinai.  Then it goes on to talk about Shmita.  Rashi asks, “Mah Inyan Shmita Eitzel Har Sinai”; what does Shmita have to do with Har Sinai.  He answers that this teaches us just like all the halachos of Shmita with all the details was given on Har Sinai so too were all the mitzvos and their intricate details.  The Chasam Sofer asks why did the Torah choose specifically the mitzva of Shmita to teach us this lesson?  

He answers that the Torah promises the farmers that even though they will not plant during shmita, Hashem will bless them with such an abundant crop the year before that it will last them for three years until the next crop is ready.  This is a mitzva that is obviously given from Sinai directly from Hashem and not invented by man, for who else would dare make such a promise other than Hashem alone.  Therefore the Torah tells us, just like Shmita was clearly given over by Hashem on Har Sinai so too were all the other mitzvos.