Musnad Ahmad 2:70

Yahya ibn Ghaylan narrated to us, Rashdin ibn Sa’d narrated to us, Abu Abdullah al-Ghafiqi narrated to me, from Zayd ibn Aslam, from his father, from Umar ibn al-Khattab, from the Messenger of Allah (may Allah bless him and grant him peace): That he performed ablution in the year of Tabuk, [washing each part] once, once.