Midrash su Levitico 25:21

R. Yossi says: It is written (Isaiah 45:19) "Not in secrecy did I speak, in a place of darkness, etc." In the very beginning, when I gave it, I did not give it in secret or in a dark, dusky land. And I did not say to the seed of Jacob "Seek Me in vain" (i.e., gratis). I did not give it as a pledge (to be taken back), viz. (Ibid.) "I am the L rd, who speaks righteousness, who declares what is just." Even before I gave you the mitzvoth I "prefaced" their reward, viz. (Exodus 16:5) "And they shall prepare what they shall bring (of the manna) and it shall be double, etc." And it is written (Leviticus 25:21) "And I shall command My blessing for you in the sixth year, etc." I might think (that reward is given) only for these (Shabbath and Shevi'ith) alone (and not for other mitzvoth). It is, therefore, written (Psalms 105:44) "And He gave them the land of nations, etc." Why? (Ibid. 45) "So that they keep His statutes and observe His laws." ...

6) (Vayikra 25:21) ("I shall command My blessing for you in the sixth year, and it shall produce its crop for three years.") (so that you will have a surplus to store away). This tells me (that there will be a surplus) only in the sixth year. Whence do I derive (the same for) the fifth, the fourth, the third, and the second? (that they will provide a surplus for the succeeding year?) From (the superfluous) "the (sixth) year" "and it shall produce its crop for three years": for the sixth, (i.e., for the time from Pesach [when the harvest begins]), for the seventh (shevi'ith), and for the end of the seventh year (until the new harvest on Pesach, on the eighth year). Variantly: "for three years": for the seventh, and Yovel, and the end of Yovel.