Reward is Proportionate to Hardship, But Deliberately Seeking Difficulty Has No Basis
As the Prophet ﷺ said, "Your reward is according to your hardship (nasab)." What is meant by hardship here is exertion — as in his ﷺ statement, "Shall I not show you that by which Allah erases sins… completing the wudu despite hardships (makarih)." This does not mean one should go out of his way to seek hardship. It was legislated for the servant simply because he needs to perform wudu — and what is meant regarding cold weather is that he does not go searching for cold water. Rather, he needed water at a time of hardship, meaning it is not prescribed for a man to seek out cold water deliberately. But if the water happens to be cold, as is the case in winter, he performs wudu with it and it does not harm him — and the reward is proportionate to the hardship endured.
As for someone who goes and places water in a refrigerator specifically to perform wudu with it so that his reward will be greater due to the hardship — this has no basis. It has no basis at all. If a person performs wudu with water as Allah created it, without doing anything extra to it, and it happens to involve hardship, then the reward is according to that hardship. This is a general principle running through everything — whether in fasting, Hajj, maintaining family ties, or other acts of worship and deeds. One should not seek out hardship for its own sake, but if hardship results from the deed, the reward for it is greater.
Similarly, if there are two paths — an easy one and a difficult one — there is no harm in taking the difficult one, because he did not seek out the difficulty for its own sake; there was simply no other way. For example, someone traveling from one place to another might go on foot instead of by car. If he goes on foot, we say his reward is proportional to the hardship — we do not say he deliberately sought out hardship, because it was not seeking difficulty for its own sake; rather he simply let nature take its course and did not seek out specific means to lessen the difficulty.
A person also has the right not to take specific measures to reduce hardship — meaning if a person wakes up at night when it is cold and performs wudu with the water that is available, there is no harm in that. But if he brings in equipment to remove the cold, such as a water heater, that is also fine, though the first case carries a greater reward, because that is the meaning of his ﷺ statement, "perfect the wudu despite hardships." Deliberately seeking out harm, however — searching for harm and difficulty for their own sake — is what the scholars prohibited. They said there is a real difference between the two situations: a person simply going along with nature even if it involves hardship, versus a person who deliberately seeks out harm or seeks out the path of hardship.