It is narrated from Sayyiduna Wa'il bin Hujr (may Allah be pleased with him) that he prayed behind the Messenger of Allah (peace and blessings be upon him), and he (peace and blessings be upon him) heard a man saying: "Al-hamdu lillahi kathiran tayyiban mubarakan fih (All praise is for Allah, abundant, pure and blessed praise)." When the Prophet (peace and blessings be upon him) finished his prayer, he said: "Who said these words?" He said: It was I, O Messenger of Allah! By Allah, I intended nothing by them but good. He (peace and blessings be upon him) said: "The gates of the heaven were opened for them, and nothing held them back short of the Throne."
Hadith Referenceالمعجم الكبير للطبراني / باب الواو / 18274