Musnad Ahmad 27:865

I read to Abd al-Rahman, [from] Malik ibn Anas, from Abdullah ibn Dinar, from Abdullah ibn Umar, that he mentioned Umar ibn al-Khattab to the Messenger of Allah, peace and blessings of Allah be upon him, [saying] that he experiences janabah (major ritual impurity) at night. The Messenger of Allah, peace and blessings of Allah be upon him, said to him: "Perform wudu (ablution), wash your private part, then sleep."