Musnad Abu Ya'la al-Mawsili 2:77

Zuhayr narrated to us, Jarir narrated to us, from Yazid ibn Abi Ziyad, from Mujahid, from Abd al-Rahman ibn Safwan, who said: I said to Umar ibn al-Khattab, "How did the Messenger of Allah (may Allah bless him and grant him peace) act when he entered Makkah?" He said, "He prayed two rak'ahs."