Talmud for Gittin 4:9
הַמּוֹכֵר אֶת עַצְמוֹ וְאֶת בָּנָיו לְגוֹי, אֵין פּוֹדִין אוֹתוֹ, אֲבָל פּוֹדִין אֶת הַבָּנִים לְאַחַר מִיתַת אֲבִיהֶן. הַמּוֹכֵר שָׂדֵהוּ לְגוֹי וְחָזַר וּלְקָחָהּ מִמֶּנּוּ יִשְׂרָאֵל, הַלּוֹקֵחַ מֵבִיא מִמֶּנּוּ בִכּוּרִים, מִפְּנֵי תִקּוּן הָעוֹלָם:
If one sold himself and his children (in servitude) to a non-Jew, he is not to be redeemed, [if he is wont to do so, as when he has already done so two or three times]; but the children are redeemed after their father's death. If one sold his field to a gentile, he buys and brings bikkurim (first-fruits) from it (see Rashi), for "the general good." [Every year he must buy its first-fruits from the gentile and bring them to Jerusalem. ("for the general good":) that he not be wont to sell land in Eretz Yisrael to gentiles, and so that, if he did sell it, he would exert himself to redeem it.]