Musnad Ahmad 27:1697

Abu al-Mughirah narrated to us, al-Awza'i narrated to us, Yahya ibn Abi Kathir narrated to us, from Abu Salamah, from Abdullah ibn Umar, that Umar ibn al-Khattab asked the Messenger of Allah, peace and blessings of Allah be upon him: "Does one of us sleep while he is in a state of janabah (major ritual impurity)?" He said: "Yes, and he performs wudu (ablution)."