Sahih Muslim Book 4, Hadith Number 1269.

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 one day deferred the prayer. ‘Urwa b. Zubair came to him and informed him that one day as Mughira b. Shu’ba was in Kufa (as its governor), he deferred the prayer, Abu Mas’ud al-Ansari came to him and said: What is this, O Mughira? Did you know that it was Gabriel who came and said prayer and (then) the Messenger of Allah (may peace be upon him) said the prayer (along with him), then (Gabriel) prayed and the Messenger of Allah (may peace be upon him) also prayed, then (Gabriel) prayed and the Messenger of Allah (may peace be upon him) also prayed, then (Gabriel) prayed and the Messenger of Allah (may peace be upon him) prayed (along with him), then Gabriel prayed and the Messenger of Allah (may peace be upon him) also prayed (along with him) and then said: This is how I have been ordered to do. ‘Umar (b. ‘Abd al-‘Aziz) said: O ‘Urwa be mindful of what you are saying that Gabriel (peace be upon him) taught the Messenger of Allah (may peace be upon him) the times of prayer. Upon this ‘Urwa said: This is how Bashir b. Abu Mas’ud narrated on the authority of his father and (also said): ‘Aisha?, the wife of the Apostle (may peace be upon him) narrated it to me that the Messenger of Allah (may peace be upon him) used to say the afternoon prayer, when the light of the sun was there in her apartment before it went out (of it).