What is the ruling on the prayer of an imam who prostrated once in a rak'ah, then stood up for the next rak'ah, and when the worshippers alerted him, he returned to prostrate without sitting between the two prostrations, according to the Maliki school of thought?

The Imam should have sat before performing the forgotten prostration, then completed his prayer and prostrated for forgetfulness after the salutation. This is because sitting is obligatory according to the Maliki school and others, and it is their relied-upon opinion. Whoever does not sit, their prayer is not valid if they do not rectify the error. It has also been said that the worshiper prostrates immediately without sitting, and according to this opinion, the prayer is not invalidated by omitting the sitting. Based on this, the Imam and those who followed him must repeat the prayer, acting upon the relied-upon opinion that the prayer is invalid in this case.