Ed. In footnote #11 to Pesachim 42a in the Artscroll Schottenstein Talmud, it states: “According to this explanation (Rashi), if the water was drawn in the morning and left to cool off all day, it would likewise be permissible to bake with it at night. The point is not that the water be kept overnight, but that it be kept a twelve-hour period (of which the night is but an example) outside of the well before using it.” Suffice it to say that the halachos of the water for matzos are surprisingly complex – including the matter of who may draw the water. For greater detail and final rulings, please consult the entire treatment of this siman with Shulchan Aruch and Mishnah Berura or the Kitzur Shulchan Aruch siman 109.
‘Mayim she-lanu’ (water which was kept overnight – abridged)MBY 455:1-6 ‘Mayim she-lanu’ (water which was kept overnight – abridged) (Excerpted from The Kleinman Edition – Kitzur Shulchan Aruch, Mesorah Publications, introduction to siman 109, p. 121) “The water used for matzos is subject to special requirements, because water that is warm can cause the dough to ferment prematurely. Now since, at the time of year when the water for matzos is drawn, the water may be somewhat warm, the water must be left standing overnight to cool off (see Mishnah Berura 455:2-3). This water is known as ‘mayim she-lanu’ (“water that has stayed overnight.”