The Prophet used to say: "Whoever performs prayer in congregation at the mosque for forty nights, never missing the first Rak'ah of the 'Isha' prayer, Allah will thereby decree for him salvation from the Fire."
عَنِ النَّبِيِّ ﷺ أَنَّهُ كَانَ يَقُولُ مَنْ صَلَّى فِي مَسْجِدٍ جَمَاعَةً أَرْبَعِينَ لَيْلَةً لاَ تَفُوتُهُ الرَّكْعَةُ الأُولَى مِنْ صَلاَةِ الْعِشَاءِ كَتَبَ اللَّهُ لَهُ بِهَا عِتْقًا مِنَ النَّارِ