whether it pertains to a Sunnah of speech or action, due to the general wording of the hadith of Ibn Mas'ud, may Allah be pleased with him, that the Prophet, peace and blessings be upon him, said: “If one of you forgets, let him prostrate twice.” (1) Moreover, if he is required to prostrate, he will be more attentive to his actions, lest he have to perform the prostration in every prayer; for these Sunnahs are commonly forgotten, especially by one who is not consistent in observing them.

What the author has mentioned—that prostration is not prescribed for its omission, yet that there is no harm if one does prostrate—indicates a beneficial principle: something may be permissible without being prescribed. That is, it may be permissible to perform it as an act of worship, without its being legislated as an act of worship. We have previously mentioned examples of this; among those that come to mind are:

First: performing an act of worship on behalf of another—for example, when someone gives charity on behalf of a deceased person. This is permissible, but it is not prescribed. In other words, we do not instruct people to give charity on behalf of their deceased relatives, because the Prophet, peace and blessings be upon him, neither instructed it nor did it himself, for it to be deemed prescribed. He did not tell the ummah: “Give charity on behalf of your deceased, fast on their behalf, pray on their behalf,” or anything similar; nor did he do so himself. The utmost that occurred is that he instructed one whose deceased relative had an outstanding fast to fast on his behalf. (2) But this concerns an obligation, and there is a difference between what is obligatory and what is not obligatory.