(18) باب صَلاَةِ الْعِشَاءِ وَالْفَجْرِ فِي جَمَاعَةٍ
It was narrated from 'Umar bin Khattab that:
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."
Reference: Sunan Ibn Majah 798
In-book reference: Book 4, Hadith 64
English translation: Vol. 1, Book 4, Hadith 798