Musnad Ahmad 2:13

Ya'qub narrated to us, my father narrated to us, from Ibn Ishaq, Nafi' narrated to me, from Abdullah ibn Umar, from his father, who said: I asked the Messenger of Allah, peace and blessings of Allah be upon him: "How should one of us act if he is in a state of major ritual impurity (junub), then wishes to sleep before he performs ghusl?" The Messenger of Allah, peace and blessings of Allah be upon him, said: "Let him perform his wudu as he does for prayer, then let him sleep."