Musnad Ahmad 27:1508

Al-Fadl ibn Dukayn narrated to us, Sufyan narrated to us, from Abdullah ibn Dinar, I heard Ibn Umar say: Umar asked the Messenger of Allah, may Allah bless him and grant him peace, and said: "I become in a state of janabah (major ritual impurity) during the night." So he commanded him to wash his private part, perform wudu (ablution), and then sleep.