"A Jewish woman entered unto me and said: 'The torment of the grave is because of urine.' I said: 'You are lying.' She said: 'No, it is true; we cut our skin and clothes because of it.' The Messenger of Allah ﷺ went out to pray and our voices became loud. He said: 'What is this?' So I told him what she had said. He said: 'She spoke the truth.' After that day he never offered any prayer but he said, following the prayer: 'Rabba Jibril wa Mika'il wa Israfil, aiding min harrin-nar wa 'adhabil-qabr (Lord of Jibril, Mika'il and Israfil, grant me refuge from the heat of the Fire and the torment of the grave).'" (Using translation from Nasāʾī 1345)   

بَلَى وَإِنَّا نَقْرِضُ مِنْهُ الْجِلْدَ وَالثَّوْبَ فَخَرَجَ رَسُولُ اللهِ ﷺ إِلَى الصَّلَاةِ وَقَدِ ارْتَفَعَتْ أَصْوَاتُنَا فَقَالَ «مَا هَذَا يَا عَائِشَةُ؟» فَأَخْبَرْتُهُ بِمَا قَالَتْ فَقَالَ «صَدَقَتْ» فَمَا صَلَّى بَعْدَ يَوْمَئِذٍ إِلَّا قَالَ فِي دُبُرِ الصَّلَاةِ «رَبَّ جِبْرِيلَ وَرَبَّ مِيكَائِيلَ وَإسْرَافِيلَ أَعِذْنِي مِنْ حَرِّ النَّارِ وَعَذَابِ الْقَبْرِ»  

The Messenger of Allah ﷺ commanded me to recite Mu'awwidhatan (the last two surahs of the Qur'an) after every prayer. (Using translation from Abū Dāʾūd 1523)  

«أَمرَنِي النَّبِيُّ ﷺ أَنْ أَقْرَأَ الْمُعَوِّذَاتِ دُبُرَ كَلِّ صَلَاةٍ»