Sahih Muslim Book 4, Hadith Number 1268.

Chapter : He who came up with the Rak’ah, he in fact came up with the prayer (led in congregation with the Imam).

Ibn Shibab reported: ‘Umar b. ‘Abd al-‘Aziz deferred the afternoon prayer somewhat and ‘Urwa said to him: Gabriel came down and he led the Messenger of Allah (may peace be upon him) in prayer. ‘Umar said to him: O ‘Urwa, are you aware of what you are saying? Upon this he (‘Urwa) said: I heard Bashir b. Abu Mas’ud say that he heard Abu Mas’ud say that he heard the Messenger of Allah (may peace be upon him) say: Gabriel came down and acted as my Imam, then I prayed with him, then I prayed with him, then I prayed with him. then I prayed with him, then I prayed with him, reckoning with his fingers five times of prayer.