Book 2, Hadith 76

Yahya related to me from Malik from Abdullah ibn Dinar that Abdullah ibn Umar related that Umar ibn al-Khattab mentioned to the Messenger of Allah, may Allah bless him and grant him peace, that he would sometimes become junub in the night. The Messenger of Allah, may Allah bless him and grant him peace, said to him, "Do wudu and wash your penis, and then sleep