Prophet Mohammad (SAW.) said that if an ill person recites the following dua forty times and dies in that illness, he will be rewarded as a martyr and if he regains his health, all his sins will be forgiven:
لَا إِلَهَ إِلَّا أَنْتَ سُبْحَانَكَ إِنِّي كُنْتُ مِنَ الظَّالِمِينَ
“There is none worthy of worship besides You O Allah, You are pure, I have oppressed my soul”.
In another hadith Prophet Mohammad (SAW.) said that if a person recites the following dua during his illness and dies, then the fire of Jahannam is forbidden for him.
(Hisnul Hasin from Trimidhi etc)
لَا إِلَهَ إِلَّا اللَّهُ وَاللَّهُ أَكْبَرُ لَا إِلَهَ إِلَّا اللَّهُ وَحْدَهُ لَا إِلَهَ إِلَّا اللَّهُ وَحْدَهُ لَا شَرِيكَ لَهُ لَا إِلَهَ إِلَّا اللَّهُ لَهُ الْمُلْكُ وَلَهُ الْحَمْدُ لَا إِلَهَ إِلَّا اللَّهُ وَلَا حَوْلَ وَلَا قُوَّةَ إِلَّا بِاللَّهِ
“There in none worthy of worship besides Allah, He is the greatest. There is none worthy of worship besides Allah, He is all by Himself.
There is none worthy of worship besides Allah. He has no partner with Him. His is the Kingdom and all Praise. He is the one to grant us the ability to save ourselves from sinning and to do good”.