We reply: that causes no harm. When you join the imam for Ẓuhr in his second rakʿah, does he not sit for the tashahhud even though, for you, it is only your first? The same principle applies here, so there is no problem.
The difficulty only appears when you come to the mosque, find the congregation praying ʿIshāʾ, and you enter with them in the very first rakʿah. In this case you will pray three rakʿahs behind the imam, after which he will rise for a fourth. What should you do then? If you rise with him you will have added a rakʿah, making four, whereas Maghrib is three, not four. If you remain seated you will fall behind the imam. So what is the solution?
We say: stay seated. If you intend to combine the prayers, formulate the intention to leave the imam, recite the tashahhud, give the salām, and then rejoin the imam for the remainder of ʿIshāʾ—you can still catch it.
If you do not intend to combine, or you are among those not permitted to do so, you now have a choice. If you wish, stay seated for the tashahhud and wait until the imam completes his rakʿah, sits, and you both end with the salām together. If you prefer, intend to separate, recite the tashahhud, and give the salām on your own.
What we have explained is the sounder opinion and was the choice of Shaykh al-Islām Ibn Taymiyyah, may Allah have mercy on him. The intention to separate here is permitted out of necessity, for a person may not perform more than three rakʿahs in Maghrib. Sitting, therefore, is required by a legitimate need, and there is no objection to it.
Included in the verse, “and you establish the prayer,” are the pillars (arkān) of the prayer—the verbal and physical acts without which the prayer is neither valid nor complete. Among these pillars is the takbīrat al-iḥrām (the opening declaration “Allāhu akbar”): the worshipper, upon entering the prayer, must say