Musnad Ahmad 32:351

Hashim ibn al-Qasim narrated to us, Ziyad ibn Abdullah ibn 'Ilathah narrated to us, Salamah ibn Wardan al-Madani narrated to us, he said: I heard Anas ibn Malik say: A man came to the Messenger of Allah, may Allah bless him and grant him peace, and said: "O Messenger of Allah, which supplication is best?" He said: "You ask your Lord for pardon and well-being in the world and the Hereafter." Then he came to him the next day and said: "O Messenger of Allah, which supplication is best?" He said: "You ask your Lord for pardon and well-being in the world and the Hereafter." Then he came to him on the third day and said: "O Messenger of Allah, which supplication is best?" He said: "You ask your Lord for pardon and well-being in the world and the Hereafter; for if you are given them in the world, then you are given them in the Hereafter, you have indeed succeeded."