The merciful prophet of Islam (pbuh) says:
لا يَزالُ المؤمنُ خائفاً من سُوءِ العاقِبَةِ، لايَتَيقّنُ الوُصولَ إلى رِضْوانِ اللَّهِ حتّى يكونَ وَقتُ نَزْعِ رُوحِهِ وظُهورِ مَلَكِ المَوتِ لَهُ.
‘The believer is continuously fearful of a bad end [to his life] and remains uncertain of attaining the pleasure of Allah until the angel of death appears and takes his soul.’
Source: Bihar al-Anwar, v. 71, p. 366, no. 13