Tosefta for Peah 6:4
וְאֵלּוּ הֵן רָאשֵׁי שׁוּרוֹת. שְׁנַיִם שֶׁהִתְחִילוּ מֵאֶמְצַע הַשּׁוּרָה, זֶה פָּנָיו לַצָּפוֹן וְזֶה פָּנָיו לַדָּרוֹם, וְשָׁכְחוּ לִפְנֵיהֶם וּלְאַחֲרֵיהֶם, אֶת שֶׁלִּפְנֵיהֶם שִׁכְחָה, וְאֶת שֶׁלְּאַחֲרֵיהֶם אֵינוֹ שִׁכְחָה. יָחִיד שֶׁהִתְחִיל מֵרֹאשׁ הַשּׁוּרָה, וְשָׁכַח לְפָנָיו וּלְאַחֲרָיו, שֶׁלְּפָנָיו אֵינוֹ שִׁכְחָה, וְשֶׁלְּאַחֲרָיו שִׁכְחָה, מִפְּנֵי שֶׁהוּא בְּבַל תָּשׁוּב (דברים כד). זֶה הַכְּלָל, כָּל שֶׁהוּא בְּבַל תָּשׁוּב, שִׁכְחָה. וְשֶׁאֵינוֹ בְּבַל תָּשׁוּב, אֵינוֹ שִׁכְחָה:
And these are the heads of rows: two who started from the middle of the row, this one facing north and that one facing south, and they forgot [some sheaves] before them and after them, the ones that are before them are <i>Shikhechah</i> and the ones that are behind them are not <i>Shikhechah</i>. An individual who starts from the head of the row and forgets [some sheaves] before himself and after himself, that which is before him is not <i>Shikhechah</i> and that which is after him is <i>Shikhechah</i>, since it falls under [the prohibition of] "Do not return." This is the general rule, all that falls under "Do not return" is <i>Shikhechah</i>, and whatever does not fall under "Do not return" is not <i>Shikhechah</i>.