A Muslim's du'a

Abdullah ibn 'Amr ibn al-'As, may Allah be pleased with him and his father, said, "Fine things, he who embodies them will be happy in this world and in the next -
the first is to regularly say, 'La ilaha illa Allah, Muhammadun rasulu Allah' - There is no god but Allah, Muhammad is the Messenger of Allah;

the second is to say when inflicted with a calamity, Inna lillahi wa inna ilayhi raji'un, wa la hawla wa la quwwata illa billahil 'aliyyil 'azim - Verily we belong to Allah and to Him we shall all return, and there is no power or might except by the will of Allah the most High, the Mighty;

the thrid is to say in gratitude when given a blessing, 'Alhamdulillahi rabbil 'alamin' - Praise be to Allah, Lorad of the Universe;

the fourth is to say when beginning anything, 'Bismillahir rahmanir rahim' - In the name of Allah the most Merciful the most Kind;

and the fifth is to say upon committing a sin, 'Astaghfiru llahal 'azima wa atubu ilayh' - I seek the forgiveness of Allah the Greatest and I repent to Him.

(Preparing for the day of Judgement by Imam ibn Hajar al-asqalani)