What is the ruling on the prayer of one who prayed Maghrib behind an Imam praying ‘Isha as a shortened prayer, believing it to be the Maghrib prayer, then the Imam salam-ed after the second rak‘ah? Should the worshipper complete a third rak‘ah and then salam? And should he recite the first and last tashahhud like the Imam, or only the first tashahhud?

Praying Maghrib behind an Imam who is praying Isha shortened is valid, and the difference in intention between the Imam and the follower does not matter, due to the generality of the Prophet's (peace be upon him) saying: "Deeds are but by intentions." The follower must recite the first and last Tashahhud if he catches the Imam in them, following him. If the follower catches the last rak'ah with the Imam, he should recite the full Tashahhud with him during the sitting for his salam, following him.