only; he is not required to perform a complete rak'ah. These are the cases of omitting pillars.
“If he forgets the first tashahhud and rises, he must return so long as he has not stood fully upright. If he has stood fully upright, it is disliked for him to return; but if he has not stood fully upright, he must return. If he has begun the recitation, it is prohibited for him to return, and he must perform the prostration of forgetfulness in all cases.”
This discussion concerns omissions of pillars. As for the obligatory acts, the author mentions them in his statement: “If he forgets the first tashahhud…” The author singled out the first tashahhud merely as an example, not by way of restriction. Rather, we say: if a person inadvertently omits an obligatory act, such as the first tashahhud, and rises, there are three possible cases:
The first case: he remembers it after rising—that is, after his thighs have separated from his calves—but before he has stood fully upright. In this case, he sits back down, recites the tashahhud, completes his prayer, and performs the prostration of forgetfulness.
The second case: he remembers it after standing fully upright, but before beginning the recitation. In this case, he does not return, because he has completely moved on from the tashahhud, having reached the pillar that follows it.
The third case: he remembers it after beginning the recitation of the following rak'ah. In this case, it is prohibited for him to return. The author has explained this distinction in his statement: “If he forgets the first tashahhud and rises, he must return so long as he has not stood fully upright. If he has stood fully upright, it is disliked for him to return; but if he has not stood fully upright, he must return. If he has begun the recitation, it is prohibited for him to return.”
His statement, “and he must perform the prostration in all cases,” means: in each of the three cases: