What is the ruling on the Maghrib and Isha prayers for one who prayed Maghrib behind an imam who was combining the Isha prayer with Maghrib in an advance combination, and when the imam stood for the fourth rak'ah, the worshiper said the taslim and joined him in the Isha prayer?

The Maliki school of thought is the most prevalent in this matter, as it permits the congregant to pray behind an imam who prays a greater or lesser number of rak'ahs, provided there is a valid reason for each of them to combine prayers. The congregant has the choice to either wait for the imam and conclude the prayer with him, which is preferable, or to intend to separate from him and conclude the prayer on his own, and there is no sin in that. This opinion is supported by some instances from the Sunnah where a congregant separated from the imam due to a need.