Reference for Yevamot 212:13
ונקרא שמו בישראל בית חלוץ הנעל מצוה בדיינין ולא בתלמידים: תניא א"ר יהודה פעם אחת היינו יושבים לפני רבי טרפון ובאתה יבמה לחלוץ ואמר לנו ענו כולכם חלוץ הנעל חלוץ הנעל חלוץ הנעל:
What is the reason? — It is necessary that she shall spit before<span class="x" onmousemove="('comment',' E.V., in. ');"><sup>54</sup></span> his face.<span class="x" onmousemove="('comment',' Deut. XXV, 9. ');"><sup>44</sup></span> If, therefore, he was tall and she was short, and the wind carried the spittle away,<span class="x" onmousemove="('comment',' V. supra note 16. ');"><sup>55</sup></span> her act is deemed to have been<span class="x" onmousemove="('comment',' Lit., 'there is'. ');"><sup>56</sup></span> before his face.<span class="x" onmousemove="('comment',' lbid., since at the moment the spittle left her mouth it was before the levir's face. ');"><sup>57</sup></span>