Question: According to the Maliki view, what is the ruling if one omits a surah after the opening fatiha in one or both rakah of salah; does this necessitate a prostration of forgetfulness? Can one minimally say 1 ayah?
Answer: If someone happens to leave out one or more of the stressed sunnah acts of the obligatory prayer it is a sunnah that one perform 2 prostrations for heedlessness before concluding the prayer with the salam.
As for the additional sura – in spite of being a stressed sunnah – one is required to do the prostrations for omitting it before concluding the prayer or shortly after if one forgets. Otherwise, the prayer is invalidated due to the omission of 3 consecutive sunnahs: the sura, standing for its recitation, and audibleness or inaudibility; whereas 3 stressed sunnahs are counted as 1 compulsory act.
As for limiting the additional recitation to one ayah, this is sufficient, although Imam Malik’s view was that one should not say only part of a sura in one rak’ah. He disapproved of this. That is, it is better for one to recite the entire sura.