Musnad Abu Ya'la al-Mawsili 154:98

Khalifah ibn Khayyat narrated to us, Yazid ibn Zuray' narrated to us, Hajjaj narrated to us, he said: Abu al-Zubayr narrated to me, from 'Awn ibn 'Abd Allah ibn 'Utbah, that he narrated to them from Ibn 'Umar, who said: While I was with the Messenger of Allah (may Allah bless him and grant him peace) in prayer, a man from the people said: "Allah is the Greatest, [much] greater; and praise be to Allah, much; and glory be to Allah, morning and evening." When the Messenger of Allah (may Allah bless him and grant him peace) finished the prayer, he said: "Which of you is the author of the words?" The people remained silent. A man from the people said: "I said them, O Messenger of Allah, and I intended nothing by them but good." The Messenger of Allah said: "The gates of the heaven were opened for them." Ibn 'Umar said: "I have not abandoned them since I heard the Messenger of Allah (may Allah bless him and grant him peace) say them."