Musnad Ahmad 2:154

'Abd al-Razzaq narrated to us, 'Ubayd Allah ibn 'Umar informed us, from Nafi', from Ibn 'Umar: That 'Umar asked the Prophet, 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, but he should perform wudu (ablution) as he does for prayer."