Reference for Pesachim 221:16
ובצילה של לבנה לא אמרן אלא במערבה אבל במדינחתא לית לן בה
What is the remedy? If there is another road [available], let one go through it. While if there is no other road, [then] if another man with him, let them clasp hands and pass through; while if there is no other man, let him say thus: 'Igrath Izlath, Asya, Belusia<span class="x" onmousemove="('comment',' The demons by whose aid you seek to work witchcraft.');"><sup>10</sup></span> have been slain with arrows.'<span class="x" onmousemove="('comment',' The text is obscure.');"><sup>11</sup></span> When one meets a woman coming up from her statutory tebillah,<span class="x" onmousemove="('comment',' After her period of menstruation.');"><sup>12</sup></span> if [subsequently] he is the first to hav intercourse, a spirit of immortality will infect him; while if she is the first to have intercourse, a spirit of immortality will infect her. What is the remedy? Let him say thus : 'He poureth contempt upon princess, and causeth them to wander in the waste, where there is no way.'<span class="x" onmousemove="('comment',' Ps. CVII, 40.');"><sup>13</sup></span> R'Isaac said: What is meant by the verse, Yea, though I walk through the valley of the shadow of death, I will fear no evil, for Thou art with me?<span class="x" onmousemove="('comment',' Ps. XXIII, 4.');"><sup>14</sup></span> This refers to him who sleeps in the shadow of a single palm-tree or in the shadow of the moon. Now in respect to the shadow of a single palm-tree this holds good only where the shadow of the neighbouring [tree] does not fall upon it; but if the shadow of the neighbouring tree falls upon it, it does not matter. Then when it was taught: He who sleeps in the shadow of a single palm-tree in a courtyard and he who sleeps in the shadow of the moon, has is blood on is own head, how is it meant? Shall we say that the shadow of the neighbouring tree does not fall upon it, - then even in a field too [it is dangerous]? Hence you may surely infer from this that in a courtyard [there is danger] even if the shadow of the neighbouring tree fall on it. This proves it. And in respect to the shadow of the moon too, this holds good only when [it falls] in the west,<span class="x" onmousemove="('comment',' I.e., at the end of the month when the moon is in the east and casts its shadow in the west.');"><sup>15</sup></span> but when it is in the east it does not matter.