Yahya related to me from Malik from Abdullah ibn Dinar thatAbdullah ibn Umar related that Umar ibn al-Khattab mentioned to theMessenger of Allah, may Allah bless him and grant him peace, that hewould sometimes become junub in the night. The Messenger of Allah, mayAllah bless him and grant him peace, said to him, "Do wudu and washyour penis, and then sleep."
USC-MSA web (English) reference: Book 2, Hadith 78
Arabic reference: Book 2, Hadith 108