Reference for Pesachim 2:3
נָכְרִי שֶׁהִלְוָה אֶת יִשְׂרָאֵל עַל חֲמֵצוֹ, אַחַר הַפֶּסַח מֻתָּר בַּהֲנָאָה. וְיִשְׂרָאֵל שֶׁהִלְוָה אֶת הַנָּכְרִי עַל חֲמֵצוֹ, אַחַר הַפֶּסַח אָסוּר בַּהֲנָאָה. חָמֵץ שֶׁנָּפְלָה עָלָיו מַפֹּלֶת, הֲרֵי הוּא כִמְבֹעָר. רַבָּן שִׁמְעוֹן בֶּן גַּמְלִיאֵל אוֹמֵר, כָּל שֶׁאֵין הַכֶּלֶב יָכוֹל לְחַפֵּשׂ אַחֲרָיו:
If a gentile lent an Israelite [money] on his (the Israelite's) chametz [before Pesach, and the Israelite said to him: "If I have not repaid you by this and this date, acquire it from now," and he left the pledge in the gentile's home, where it remained all of Pesach], after Pesach, he may derive benefit from it. [For since the time arrived and he did not pay him, the chametz, being in the gentile's domain, did not lack "claiming," so that it is seen retroactively that at the time he pledged it with him, it was his (the gentile's)]. And if an Israelite lent a gentile on his (the gentile's) chametz — after Pesach, he may not derive benefit from it. [For it is seen retroactively that it was the Israelite's.] If debris falls upon chametz, it is considered removed, [notwithstanding which he must nullify it, lest the mound be removed on the festival and he be found to have transgressed.] R. Shimon b. Gamliel says: Whatever a dog cannot search after (is considered removed). [How much can a dog search after? (A distance of) three handbreadths.]