Reference for Bava Metzia 6:4
הַשּׂוֹכֵר אֶת הַפָּרָה לַחֲרשׁ בָּהָר וְחָרַשׁ בַּבִּקְעָה, אִם נִשְׁבַּר הַקַּנְקַן, פָּטוּר. בַּבִּקְעָה וְחָרַשׁ בָּהָר, אִם נִשְׁבַּר הַקַּנְקַן, חַיָּב. לָדוּשׁ בַּקִּטְנִית וְדָשׁ בַּתְּבוּאָה, פָּטוּר, לָדוּשׁ בַּתְּבוּאָה וְדָשׁ בַּקִּטְנִית, חַיָּב, מִפְּנֵי שֶׁהַקִּטְנִית מַחֲלָקֶת:
If one hired a cow to plow on a mountain [(and all of the owner's plowing implements and his youths go with his animal and plow with it)], and he plowed in a valley — If the coulter broke, he is not liable. (If he hired it to plow in a valley) and he plowed on a mountain — if the coulter broke, he is liable, [for mountains, being rocky, are harder to plow than valleys.] To tread pulse, and it trod grain, he is not liable [if it slipped]. To tread grain, and it trod pulse, he is liable, for pulse is slippery.