According to Mālik and his companions, a prostration of forgetfulness is due for any slight addition that occurs in the prayer, even when the added element is not of the same kind as the prayer itself.

You should also know that, in their terminology, both the Sunnah and the raghībah fall under the heading of what is recommended (mandūb); they differ only in degree—that is, in how strongly the command is emphasized. The degree of emphasis is inferred from the contextual indicators that surround each act of worship. Because of this, disagreement over matters in this category is frequent. Some jurists even maintain that certain Sunan—if deliberately omitted when they consist of actions, or deliberately performed when their very omission is the Sunnah—take on the ruling of an obligation, meaning that one would incur sin. This view is common among the followers of Mālik.

Likewise, all the schools—apart from the Ẓāhirīs—agree that a person who persistently abandons the regularly repeated Sunan is sinful. Thus, if someone were always to omit Witr or the two rakʿahs before dawn, he would be deemed sinful and morally deficient. Viewed in this way, acts of worship divide into: (1) those that are obligatory both in themselves and in their type, such as the five daily prayers; (2) those that are Sunnah in themselves yet obligatory in type, such as Witr and the two rakʿahs of Fajr; and (3) in the opinion of some scholars, certain raghāʾib that remain raghāʾib in themselves while counting as Sunan in type. An example is the Mālikī position—mentioned earlier—that obliges a prostration for omitting more than one takbīrah, i.e., when they are forgotten. In my judgment, such acts are not considered Sunan in both essence and type by those scholars.

The Ẓāhirīs, however, regard Sunan simply as Sunan, citing the Prophet’s—peace and blessings be upon him—words to the Bedouin who asked about the obligations of Islam: “He will succeed if he is truthful; he will enter Paradise if he is truthful.” He said this after the man affirmed, “By Allah, I will add nothing to this and take nothing away from it,” meaning the prescribed obligations alone. This ḥadīth has already been cited.

All the jurists agree that forgetting the middle sitting requires a prostration of forgetfulness, but they differ over whether that sitting itself is obligatory or Sunnah. They also disagree about whether the imam should return to it if the congregation alerts him with tasbīḥ, and—if he is to return—when he should do so. The majority say he returns so long as he has not yet stood fully upright. Others say he returns so long as he has not commenced the third rakʿah. Still others say he does not return once he has risen a handspan from the ground.

If, according to those who believe he should not return, the imam nevertheless does so, the majority still judge his prayer valid, whereas some declare it invalid.

[Chapter Four: The Manner of Performing the Prostration of Forgetfulness]

As for how the prostration of forgetfulness is performed, the scholars disagreed. Mālik held that when the two prostrations come after the closing salām, one should recite the tashahhud within them and conclude with another salām; Abū Ḥanīfah concurred, for in his view all prostrations of forgetfulness take place after the salām. When the prostrations precede the salām, Mālik maintained that one recites the tashahhud for them only and that the salām ending the prayer serves for them as well; al-Shāfiʿī held the same view, since for him all such prostrations are before the salām. It is also reported from Mālik that no tashahhud is required for the prostrations made before the salām, and a group of scholars agreed with him.