What the prohibitive form entails:
When left unrestricted, the prohibitive form indicates two things about the act it targets: it is forbidden and it is invalid.
Evidence that it entails prohibition is found in the Almighty’s words: “Whatever the Messenger gives you, take it; and whatever he forbids you from, abstain” (al-Hashr 59:7). The command to abstain from what he has forbidden makes refraining obligatory, and that necessarily means the act itself is prohibited.
Evidence that it entails invalidity is the Prophet’s ﷺ saying, “Whoever performs an act that is not in accordance with our affair is rejected.” (1) That is, it is null and void. Anything the Prophet ﷺ has prohibited is, by definition, not in accordance with his guidance, and is therefore rejected.
The school’s guiding principle regarding a prohibited act—whether it is null or merely sinful—runs as follows:
1 - If the prohibition bears on the very substance of the act or on one of its conditions, the act is null and void.
2 - If the prohibition concerns an external factor unrelated to the substance of the act or its conditions, the act is not null.