It is also possible to distinguish between a passer whose passing invalidates the prayer and one whose passing does not invalidate it. The former must be prevented, whereas the latter need not be prevented, because the most that results from his passing is that the prayer is diminished, not invalidated. This differs from one whose passing invalidates the prayer, for he will invalidate your prayer and spoil it for you—especially if it is an obligatory prayer. Allowing someone to invalidate your obligatory prayer means that you have cut short your obligatory prayer, and the basic ruling on interrupting an obligatory prayer is that it is prohibited.
This is a middle position between the view of those who hold that preventing the passer is obligatory in all cases and the view of those who hold that it is recommended in all cases. It is a strong view.
For example, if a woman passes by, you must prevent her; if a black dog passes by, you must prevent it; and if a donkey passes by, you must prevent it. In contrast, if a man passes by, or an animal other than a donkey, or a dog that is not black, or a female below the age of puberty, then preventing them is not obligatory, though it is recommended.
It is also possible to distinguish between an obligatory prayer and a voluntary prayer. Thus, if the prayer is obligatory and someone whose passing invalidates it passes by, he must be prevented, because once one has begun an obligatory prayer, it is forbidden to interrupt it except out of necessity. Otherwise, preventing him is not obligatory, though it is recommended. This type of qualification between the two views often appears in the words of Shaykh al-Islam, may Allah have mercy on him. He says: “It is part of the view of those who hold it to be obligatory,” or something similar. An example is his statement concerning witr: Witr is obligatory for one who has a regular portion of prayer at night. He said: “It is part of the view of those who hold it to be obligatory without qualification,” because the scholars have three views regarding witr: that it is recommended without qualification—which is the correct view—and that it is obligatory without qualification,