Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates that the Prophet (peace and blessings be upon him) said: "No remembrance is more excellent than 'La ilaha illallah' (There is no deity except Allah), and no supplication is more excellent than 'Astaghfirullah' (I seek forgiveness from Allah)." Then the Prophet (peace and blessings be upon him) recited this verse: "Know that there is no deity except Allah Almighty, and seek forgiveness for your sin and for the believing men and the believing women."
Tabarani recorded it; its chain contains al-Ifriqi and other da'if (weak) narrators.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الأذكار / 16817