Musnad Ahmad 2:68

Hassan narrated to us, Ibn Lahi'ah narrated to us, al-Dahhak ibn Sharhabil narrated to us, from Zayd ibn Aslam, from his father, from Umar ibn al-Khattab, that he said: I saw the Messenger of Allah, peace and blessings of Allah be upon him, perform ablution once for each part.