Yosef Karo

“ Paragraph 4 - Regarding the wheat that is used to make the matzah used to fulfill the commandment, it is good to protect it such that no water falls on it from the time of harvesting, or at least from the time of grinding. In a pressing scenario, it is permitted to get flour from the market. ”

“ Great men will go themselves to the mill to see their flour being ground for themselves (Mordechai) .
סעיף ח - נוהגין לנקר הרחים, משום דזמנין נתנו בהם תבואה לתותה לסלת. ונוהגים שקמח הראשון שנטחן אחר הנקור שומרים אותו עד לאחר הרגל. הגה: ואנשי מעשה רגילים לילך בעצמם אל מקום הרחים לראות הם בעצמם לטחינת קמחיהם (מרדכי) .
Paragraph 9 - The wheat must be ground a day or two before the kneading. If it is ground on the day before Passover, some say that it is forbidden to knead it into matzah that day, because during grinding, the wheat gets hot and can heat the water and then the dough may ferment. ”

“ However, it is obvious that they do not render something forbidden after the fact if they fall into the pot. Similarly, it is permissible to light using oil made from them, and it does not render something forbidden if it falls into the dish. Similarly, it is permitted to keep legumes in the house [during Passover] (Trumat Hadeshen chapter 113) . The seed of dill (Mahari"l) , anise, and coriander are not legumes and it is permitted to eat them on Passover. ”