بسم الله الرحمن الرحيم
(Fatwa: 1135/905/D=10/1434)
It is wajib to offer fardh salah with jamah in the mosque. Those who offer Isha salah with jamah at home they are deprived of the virtues of mosque. It is heinous to do such a job which deprives one of the reward and virtues in the holy month of Ramadan which is meant for availing virtues and rewards more and more. However, Isha salah shall be valid and they shall have the sin of not attending the mosque.
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband