Narrated Omran ibn Husayn that Allah's Messenger, Peace Be upon Him said: "The best of Allah's servants on the Day of Resurrection are al-Hamdoun 'the ones who praise Allah.'"
Al-Muj'am al-Kabir by al-Tabarani (18/124), Classed as Saheeh by al-Albanin in Saheeh al-Jami' (1571) and al-Sahiha (1584).