Even if he regrets it while prostrating, then stands and performs the bowing, it will not benefit him, because the prayer becomes invalid as soon as he omits it. He must therefore repeat the prayer anew.

An example of omitting an obligation is if one deliberately omits the first tashahhud until he has stood up, then regrets it and returns. His prayer is invalid, even if he returns, because he deliberately omitted it. Whenever one deliberately omits an obligation, his prayer is invalid.

Unlike the rest, …

His statement, “Unlike the rest,” means: apart from the conditions, pillars, and obligations. The prayer is not invalidated by omitting the rest, even deliberately, because they are Sunnahs that complete the prayer. If they are observed, the prayer is more complete; if they are absent, the prayer is diminished, but this is a deficiency in completeness, not a deficiency affecting its validity.

Everything else consists of verbal and physical Sunnahs for whose omission no prostration is prescribed; though if one does prostrate, there is no harm …

His statement, “Everything else consists of verbal and physical Sunnahs,” means: everything other than the pillars and obligations of the prayer. The word “everything” here means “that which”; that is, that which is beyond these matters. “Beyond” means: that which goes beyond them.

Verbal Sunnahs are those statements which are recommended to say, and physical Sunnahs are those actions which are recommended to perform.

The meaning of Sunnah among the jurists, may Allah have mercy on them, differs from its meaning in the terminology of the Companions and the Successors. In their terminology, Sunnah means a prescribed way, and it may be obligatory or recommended. Thus, Anas ibn Malik said, for example: “It is from the Sunnah that, if a man marries a virgin while married to a previously married woman, he stays with her for seven nights, then apportions his time equally.” (1) Here, Sunnah means what is obligatory.