Reference for Yevamot 68:3

כמעשה ער ואונן ולא כמעשה ער ואונן כמעשה ער ואונן דכתיב (בראשית לח, ט) והיה אם בא אל אשת אחיו ושחת ארצה ולא כמעשה ער ואונן דאילו התם שלא כדרכה והכא כדרכה

one may thresh within and winnow without;<span class="x" onmousemove="('comment',' Euphemism. This would prevent possible conception which might deprive the young child of the breast feeding of his mother. ');">⁶</span> these are the words of R. Eliezer. The others said to him: Such actions are only like the practice of Er and Onan!<span class="x" onmousemove="('comment',' Which implies that there was natural contact. Cf. supra note 5. ');">⁷</span> -Like the practice of Er and Onan, and yet not [exactly] like the practice of Er and Onan: 'Like the practice of Er and Onan', for it is written in Scripture, And it came to pass, when he went in unto his brother's wife, that he spilt it on the ground;<span class="x" onmousemove="('comment',' Gen. XXXVIII, 9. ');">⁸</span>

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