Musnad Ahmad 27:215

Yahya ibn Sa'id narrated to us, from 'Ubayd Allah, Nafi' informed me, from Ibn 'Umar, that 'Umar 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 major ritual impurity (junub)?" He said: "Yes, if he performs ablution."