It is narrated from Sayyiduna Abdullah bin Amr (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "There is no remembrance better than La ilaha illallah (there is no god but Allah), and no supplication better than seeking forgiveness." Then the Messenger of Allah (peace and blessings be upon him) recited this verse: "So know that there is no god but Allah, and seek forgiveness for your sin and for the believing men and the believing women" [Surah Muhammad: 19].
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13726