(Exodus 23:10) "Six years shall you sow your land": R. Eliezer says: When Israel does the L–rd's will, they observe one shemitah (fallow year) in a seven-year period, as it is written "And six years shall you sow your land." And when they do not do His will, they observe four shemitoth in one seven-year period. How so? (The land is left) fallow one year and sown the next year, etc., so that they are found to observe four shemitoth in one seven-year period.