Sayyiduna Harithah bin Wahb al-Khuza'i (may Allah be pleased with him), whose mother was married to Sayyiduna Umar (may Allah be pleased with him) and bore him Ubaydullah, said: "I prayed two rak'ahs behind the Messenger of Allah (peace and blessings be upon him) at Mina during the Farewell Hajj, when the people were at their greatest number."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 3230