Reference for Bava Batra 125:4

תנו רבנן בן לוי שמכר שדה לישראל ואמר לו על מנת שמעשר ראשון שלי מעשר ראשון שלו ואם אמר לי ולבניי מת יתן לבניו

[If the man says], Give him a share [in the cistern] for his pail,<span class="x" onmousemove="('comment',' I.e., for watering his cattle and not his field, for which at the utmost only half the cistern is required. Hence the gift is at the utmost only half of a half, and we strike the balance between this and a fraction. ');">⁷</span> he is to receive not less than an eighth. [If he says, Give him a share] for his pot,<span class="x" onmousemove="('comment',' For purposes of cooking, for which only a third of the cistern is required. ');">⁸</span>

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