Answer
(Fatwa: 429/240/D=1432)
According to Ahnaf, it is not lawful to offer any salah before its time combining with other salah even in case of excuse too. Hence, in the said case you should not join them in Isha salah rather you should offer Isha salah in its time. If there is no way to offer it with jama’ah, offer it alone.
إن تاخير الصلاة عن وقتها من الكبائر ؛ فلا يباح بعذر السفر والمطر كسائرالكبائر عن ابن عباس: من جمع بين صلاتين فى وقت واحد فقد أتى باباً من الكبائر (اخرجه الترمذى والحاكم) بدائع 1/328 زكريا
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband