Parshas Tazria/Metzora
The chazal bring down a possuk ‘Tain chelek lishiva vigam lishmona’. They explain this means if you keep the mitzvah of niddah which is seven days, you will have a boy which has his circumcision on the eighth day. What then does the end of the possuk refer to by saying ‘ki lo saida ma raa yiheyeh baretz’, since you do not know what bad will happen in the land?
The Ksav Sofer explains, even though there are reasons for mitzvohs and you can do the mitzvah of niddah for practical reasons still the proper way is to do it because Hashem commanded. The possuk ends with ‘since you do not know what bad will happen’ so therefore you are not doing it because of fear of bad things happening.