Musnad Ahmad 27:741

Yahya narrated to us, from Sufyan, Abdullah ibn Dinar narrated to us, I heard Ibn Umar say: Umar asked the Messenger of Allah, peace and blessings of Allah be upon him, saying: "I become in a state of janabah (major ritual impurity) during the night?" So he commanded him to wash his private part and to perform wudu (ablution).