Musnad Ahmad 2:24

Ubaydah ibn Humayd narrated to us, Ubayd Allah ibn Umar narrated to me, from Nafi, from Ibn Umar, from Umar ibn al-Khattab, who said: The Messenger of Allah (peace and blessings of Allah be upon him) was asked: "May a man sleep while he is in a state of major ritual impurity (junub)?" He said: "Yes, if he performs ablution."