In Vayikra 25: 20-22 (below) God says that he will guarantee a bountiful harvest if the Shmita is observed. Unlike other promises of spiritual reward, this one is easily quantifiable with observable ...
Rashi says What does the subject of Shemittah have with Mount Sinai? Were not all the commandments stated from Sinai? However, [this teaches us that] just as with Shemittah, its general principles ...