Mishkat al-Masabih 4 Prayer كتاب الصلاة 561

وَعَنْ أَنَسٍ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: ” مَنْ صَلَّى لِلَّهِ أَرْبَعِينَ يَوْمًا فِي جَمَاعَةٍ يُدْرِكُ التَّكْبِيرَةَ الْأُولَى كُتِبَ لَهُ بَرَاءَتَانِ: بَرَاءَةٌ مِنَ النَّارِ وَبَرَاءَةٌ مِنَ النِّفَاق ” . رَوَاهُ التِّرْمِذِيّ

Anas reported God’s Messenger as saying, “If anyone prays to God for forty days in congregation being present in time for the first takbira , two immunities will be recorded for him, one from hell and one from hypocrisy,” Tirmidhi transmitted it.