Reference for Yevamot 151:3
שלח ליה רבא בריה דרבה לרב יוסף ילמדנו רבינו היכי עבדינן א"ל מייתינן נהמא חמימא דשערי ומנחינן ליה אבי פוקרי ומקרי וחזינן ליה
The other replied: Warm barley bread is procured, and placed upon the man's anus. Thereby the flow of semen sets in, and the effect can be observed. Said Abaye: Is everybody like our father Jacob concerning whom it is written, My might, and the first-fruits of my strength,<span class="x" onmousemove="('comment',' Gen. XLIX, 3, referring to Reuben, Jacob's firstborn son. ');"><sup>4</sup></span> because he never before experienced the emission of semen!<span class="x" onmousemove="('comment',' Other people are not so saintly. Why then should the elaborate test described be necessary in ordinary cases? ');"><sup>5</sup></span> — No, said Abaye, coloured garments<span class="x" onmousemove="('comment',' Peculiar to women. ');"><sup>6</sup></span>