Musnad Ahmad 2:277

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from ‘Abdullah ibn Dinar, who said: I heard Ibn ‘Umar saying: ‘Umar asked the Messenger of Allah, may Allah bless him and grant him peace, and said: "I become in a state of major ritual impurity (janabah) during the night, so what should I do?" He said: "Wash your private part, then perform ablution (wudu), then sleep."