We performed Asr Prayer behind the Imam but after the 4th rak’ah he stood and performed a 5th rak’ah, on Muswalli alerted him but he continued. Some of the congregation stood with the Imam and others remained seated. One Muswalli then said: follow the Imam, so some people stood and others remained seated till the Imam made sajdah sahw and made taslim, those who remained seated made talsim with the Imam bearing in mind that some people made taslim before Imam and others left the prayer. When Prayer ended the Imam was asked: why did ypu make the 5th rak’ah and he replied them: it is because I forgot to read surah al-Fatihah on the 2nd rak’ah. At this point some congregants said: Prayer is correct for those eho did not stood up with the Imam and invalid for those who stood with the Imam. And when the person who said: follow the Imam was asked why he did that? He replied by saying it is compulsory to follow the Imam in every situation from the first takbir to when the prayers ends because this is supported by an authentic hadith which instruct that.
زاد الإمام ركعة فما الحكم؟