Hadith 1071
'Uthman bin 'Affan (May Allah be pleased with him) reported:
عن عثمان بن عفان رضي الله عنه قال: سمعت رسول الله صلى الله عليه وسلم يقول: "من صلى العشاء في جماعة، فكأنما قام نصف الليل ومن صلى الصبح في جماعة، فكانما صلى الليل كله" ((رواه مسلم)). وفي رواية الترمذي عن عثمان بن عفان رضي الله عنه قال: قال رسول الله صلى الله عليه وسلم : "من شهد العشاء في جماعة كان له قيام نصف ليلة، ومن شهد العشاء والفجر في جماعة، كان له كقيام ليلة" ((قال الترمذي حديث حسن صحيح))
I heard the Messenger of Allah (ﷺ) saying: "One who performs 'Isha' prayer in congregation, is as if he has performed Salat for half of the night. And one who performs the Fajr prayer in congregation, is as if he has performed Salat the whole night." [Muslim] The narration of At-Tirmidhi says: 'Uthman bin Affan (May Allah be pleased with him) narrated that he had heard the Messenger of Allah (ﷺ) saying: "He who attends 'Isha' in congregation, is as if he has performed Salat for half of the night; and he who attends 'Isha' and Fajr prayers in congregation, is as if he has performed Salat for the whole night."
In plain words
Praying 'Isha' in congregation is like standing half the night in prayer, and joining both 'Isha' and Fajr in congregation is like praying the whole night, so these two carry great reward.
Reference : Riyad as-Salihin 1071 In-book reference : Book 8, Hadith 81