Hadith 4:87
‘Ubaidallah b Khalid reported God’s messenger as saying, “Sudden death is an act of punishment out of anger.” Abu Dawud transmitted it. Baihaqi in Shu'ab al-iman and Razin in his book made an addition, saying “An act of punishment out of anger for the infidel and a mercy for the believer.”
وَعَنْ عُبَيْدِ اللَّهِ بْنِ خَالِدٍ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «مَوْتُ الْفُجَاءَة أَخْذَةُ الْأَسَفِ» . رَوَاهُ أَبُو دَاوُدَ وَزَادَ الْبَيْهَقِيُّ فِي شُعَبِ الْإِيمَانِ وَرَزِينٌ فِي كِتَابِهِ: «أَخْذَةُ الأسف للْكَافِرِ وَرَحْمَة لِلْمُؤمنِ»
Original Reference: Mishkat al-Masabih 1611