Is it permissible for a worshipper to combine the Maghrib and Isha prayers if he catches the Imam in the last sitting of the Maghrib prayer during rain-induced combination?

It is permissible to combine prayers if you intended it at the time of consecration (ihram) for the Maghrib prayer or during its performance before concluding it. Some scholars hold the view that combining prayers is permissible even for those who intended it after completing the first prayer, and this is the chosen opinion of Sheikh al-Islam Ibn Taymiyyah and Ibn Uthaymeen. However, it is more cautious not to combine if the intention was not made until after the salutation (taslim), to avoid scholarly disagreement. Realizing less than one rak'ah with the Imam does not prevent combining prayers.