Musnad Ahmad 2:149

'Abdullah ibn Numayr narrated to us, 'Ubaydullah narrated to us, from Nafi', from Ibn 'Umar, from 'Umar, who said: I said, "O Messenger of Allah, may one of us sleep while he is in a state of janabah (major ritual impurity)?" He said, "Yes, if he performs wudu (ablution)."