סימן קלו סעיף ז
צָרִיךְ לִזָּהֵר שֶלֹּא יִקְצֹץ הַיִשְֹרָאֵל בְּעַצְמוֹ מִן הָאִילָן אֶחָד מֵאַרְבַּעַת הַמִּנִים לְצָרְכּוֹ, אֲפִלּוּ נָתַן לוֹ בַּעַל הַקַרְקַע רְשׁוּת, אֶלָּא גּוֹי אוֹ יִשְֹרָאֵל אַחֵר יִקְצְצֵם וְיִקַּח מִמֶּנוּ.
Care should be taken that a Jew should not personally cut off any of the four species from a tree for his own use, even if the owner of the land gives him permission;4This halachah is based on the concern that the non-Jew possessed the land illegally by stealing it from its Jewish owner. Therefore if you personally take these species from the land, you may be actually acquiring them illegally. It is best therefore that you acquire them from a non-Jew who took them from the land and possessed them, for once he becomes the owner, you may legally purchase them from him. Mishnah Berurah questions if this ruling applies to our times. (See Biur Halachah 649:1 and Mishnah Berurah 649:10) but a non-Jew or another Jew should cut them off and then he should buy it from him.
תנא דבי אליהו כל השונה הלכות בכל יום מובטח לו שהוא בן העולם הבא שנאמר הליכות עולם לו אל תקרי הליכות אלא הלכות (נדה ע"ג.)