Sunan Ibn Majah 798
سنن ابن ماجه#798Mawdu'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
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Al-AlbaniMawdu
Muhammad Fouad Abd al-BaqiHasan
Zubair Ali ZaiDaif
In-book reference: 4:64