It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him): the Prophet (peace and blessings be upon him) said: "When a person says: 'Jazak Allahu khayran (May Allah reward you with good),' he has given the fullest praise."
Al-Tabarani recorded it in al-Mu'jam al-Saghir; its chain contains a narrator, Musa bin Ubaydah al-Rabadhi, who is da'if (weak).