Musnad Ahmad 2:225

Yazid narrated to us, Muhammad ibn Ishaq informed us, from Nafi’, from Ibn ‘Umar, from ‘Umar ibn al-Khattab, who said: I asked the Messenger of Allah, peace and blessings of Allah be upon him, and said: "O Messenger of Allah, when one of us wishes to sleep while he is in a state of major ritual impurity (junub), what should he do before he performs ghusl?" He said: "He should perform his wudu as he does for prayer, then he may sleep."