The ruling on entering the mosque after the completion of the 'Isha' prayer and before the tarawih prayer?

Question : The questioner says: We entered the mosque, three of us, and found that the imam had finished the 'Isha' prayer. Is it better for us to pray 'Isha' in congregation, or should we wait for the night prayer and pray behind the imam with the intention of 'Isha', and then complete the rest of the 'Isha' prayer?

Answer : It is better for the one who enters while the imam is in the night prayer and he has not prayed 'Isha'—and by the night prayer we mean the tarawih prayer, because it is the one that takes place at the beginning of the time and during the time of 'Isha'—that if a group enters and the imam is praying tarawih, it is better for them to pray the 'Isha' prayer, but they should be in a place away from disturbing the worshippers. Then, when they finish the 'Isha' prayer, they join the imam. However, if only one man enters and the imam is praying tarawih, and this man has not prayed 'Isha', he joins the imam and prays behind him with the intention of 'Isha'. For example, if he catches the imam in the first rak'ah of tarawih, then when the imam says the taslim, this man performs two rak'ahs. And there is no merit in that.
And if he enters with him in the second rak'ah, then when the imam says the taslim, he performs three rak'ahs. Imam Ahmad (may Allah have mercy on him) explicitly stated the permissibility of this issue.