Musnad Ahmad 27:993

'Affan narrated to us, 'Abd al-'Aziz bin Muslim narrated to us, 'Abdullah bin Dinar narrated to us, from 'Abdullah bin 'Umar, that 'Umar mentioned to the Messenger of Allah, peace and blessings of Allah be upon him, that he would become in a state of janabah (major ritual impurity) during the night. The Messenger of Allah, peace and blessings of Allah be upon him, commanded him to wash his private part and perform wudu (ablution), then sleep.