When does prohibition imply invalidity?

The questioner : Now, Shaykh (may Allah preserve you), when does prohibition imply invalidity?

The Shaykh : When does prohibition imply invalidity? This is a matter of extreme subtlety. The conclusion I have reached over many years ago is that if the prohibition is directed at the entity in which the prohibited act exists, then this is evidence that the prohibition implies invalidity. However, if the prohibition is directed at the act itself...
Abu Malik: Peace be upon you.
The Shaykh: And upon you be peace, and the mercy of Allah and His blessings. When a text prohibits something in relation to something else, the prohibition of that thing does not imply the invalidity of the other thing. But if the prohibition of the other thing is because it contains the first thing, then it implies invalidity. An example is the well-known hadith: This is a prohibition of prayer without recitation. But similar to it is: This is an example not transmitted from the Messenger of Allah (peace and blessings be upon him) , and I will mention the transmitted one . But if this were authentic, it would be evidence for the invalidity of the prayer of one who prays in a shortened manner. However, the prohibition of shortening in prayer has come. If a man prays in a shortened manner, does the prohibition here imply invalidity? If the text had prohibited prayer in a shortened manner, it would be evidence for the invalidity of the prayer. But the prohibition is the opposite: it prohibits shortening in the prayer. This does not imply the invalidity of the prayer. I mean, in the case of something in something else, what is the prohibited thing here? It is the shortening. What is the thing in which this shortening occurred? It is the prayer. Does this imply the invalidity of the prayer? The answer is no. If the prohibition had prohibited prayer with shortening, it would imply the invalidity of the prayer. But since the prohibition prohibits shortening in the prayer, this does not imply the invalidity of the prayer, but rather it implies the sin of the one who prays in a shortened manner in his prayer. From here, we arrive at understanding many branch issues based on this principle of jurisprudence: it is not merely the prohibition that implies the invalidity of what the prohibition falls upon, but rather, if what the prohibition falls upon is prohibited because of what occurs in it, then the prohibition implies invalidity. From here, we arrive at, for example, prayer in usurped land. Does prayer in usurped land imply invalidity and corruption? First, there is no text prohibiting prayer in usurped land. But this is a branch issue addressed in the schools of jurisprudence, and some of them held that the prayer is invalid. But those who hold that the prayer is invalid have no evidence other than that he prayed in land that is usurped. Usurpation is prohibited in itself, whether for residence or for prayer. So the prohibition was not of the prayer in itself, but of the usurpation that may have befallen it. Therefore, this implies the sin of the one who prays in usurped land, but it does not imply the invalidity of this prayer that occurred in usurped land. This is what clarifies for me the answer to that question.