It is narrated from Hazrat Anas (may Allah be pleased with him) that a man came to the service of the Prophet (peace and blessings be upon him) three times and asked this question three times: "O Messenger of Allah (peace and blessings be upon him)! Which supplication is the best?" And the Prophet (peace and blessings be upon him) gave the same answer all three times: "Ask your Lord for forgiveness and well-being in this world." And the last time he said: "If you are granted these two things in this world and the Hereafter, then you have succeeded."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 12291
Hadith Gradingحکم دارالسلام:حسن لغيره، وهذا إسناد ضعيف لضعف سلمة بن وردان المدني