"But the seventh year you shall release it and let it lie fallow" (Exodus 23:11). "You shall release it" means from weeding; "and let it lie fallow" means from clearing away stones. There are two kinds of weeding. Stopping up cracks [in the ground] is permitted; strengthening the trees is forbidden. A certain heretic said to Rabbi Abbahu: Your God is a mocker, for He said to Ezekiel, "Lie on your left side" (Ezekiel 4:4), and it is written, "and you shall lie on your right side" (Ezekiel 4:6) [as if He were toying with the prophet]. Meanwhile, in came a certain disciple who said to him: What is the reason for the seventh year? He said to him: Now I will tell you something that will serve the two of you together. The Holy One, blessed be He, said to Israel: Sow for six, and release the seventh, so that you may know that the land is Mine. But they did not do so; rather, they sinned and were exiled. It is the way of the world that when a province rebels against a king of flesh and blood, if he is cruel he kills them all; if he is merciful he kills half; and if he is full of mercy, he afflicts the great ones among them with sufferings. So too the Holy One, blessed be He, afflicts the righteous with sufferings, in order to scour away the iniquities of Israel.
Rabbi Abbahu and the Heretic on Why the Land Must Rest
Yalkut Shimoni on Torah 354:2
וְהַשְּׁבִיעִת תִּשְׁמְטֶנָּהּ וּנְטַשְׁתָּהּ, ["תִּשְׁמְטֶנָּהּ"], מִלְקַשְׁקֵשׁ "וּנְטַשְׁתָּהּ", מִלְּסַקֵּל, תְּרֵי קִשְׁקוּשֵׁי הָווּ. סְתּוּמֵי פִילֵי, שָׁרֵי. אַבְרוּיֵי אִילָנֵי, אָסוּר. אֲמַר לֵיהּ הַהוּא מִינָאָה לְרַבִּי אַבָּהוּ. אֱלֹהֵיכֶם גַּחֲכָן הוּא, דְּקָאֲמַר לִיחֶזְקֵאל (יחזקאל ד, ד) "שְׁכַב עַל צִדְּךָ הַשְּׂמָאלִי", וּכְתִיב (שם, ו) "וְשָׁכַבְתָּ עַל צִדְּךָ הַיְמָנִי". אֲדְהָכֵי וְהָכֵי אֲתָא הַהוּא תַלְמִידָא, אֲמַר לֵיהּ, מַאי טַעְמָא דִּשְׁבִיעִית, אֲמַר לֵיהּ, הָשַׁתָּא אֲמִינָא לְכוּ מִלְּתָא דְּשָׁוְיָא לְתַרְוַיְיכוּ. אֲמַר הַקָּדוֹשׁ בָּרוּךְ הוּא לְיִשְׂרָאֵל, זִרְעוּ שֵׁשׁ, וְהַשְׁמִיטוּ שְׁבִיעִית, כְּדֵי שֶׁתֵּדְעוּ שֶׁהָאָרֶץ שֶׁלִּי. וְהֵם לֹא עָשׂוּ כֵן, אֶלָּא חָטְאוּ וְגָלוּ. מִנְהָגוֹ שֶׁל עוֹלָם, מֶלֶךְ בָּשָׂר וָדָם, שֶׁסָּרְחָה עָלָיו מְדִינָה, אִם אַכְזָרִי הוּא הוֹרֵג אֶת כֻּלָּהּ, וְאִם רַחֲמָן הוּא, הוֹרֵג אֶת חֶצְיָהּ, וְאִם רַחֲמָן מָלֵא רַחֲמִים הוּא, מְיַסֵּר אֶת הַגְּדוֹלִים בְּיִסּוּרִין. אַף הַקָּדוֹשׁ בָּרוּךְ הוּא מְיַסֵּר אֶת הַצַּדִּיקִים, כְּדֵי לְמָרֵק עֲוֹנֹתֵיהֶם שֶׁל יִשְׂרָאֵל.