Sunan Ibn Majah, Chapter: The Chapters on Supplication

"A man come to the Prophet (saw) and said: 'O Messenger of Allah, what supplication is best?' He said: 'Ask your Lord for forgiveness and to be kept safe and sound in this world and in the Hereafter.' Then (the man) came the next day and said: 'O Messenger of Allah, what supplication is best?' He said: 'Ask your Lord for forgiveness and to be kept safe and sound in this world and in the Hereafter.' Then (the man) came the third day and said: 'O Prophet of Allah, what supplication is best?' He said: 'Ask your Lord for forgiveness and to be kept safe and sound in this world and in the Hereafter, for if you are forgiven and kept safe and sound in this world and the Hereafter, you will have succeeded.'"