Musnad Ahmad 27:480

'Abd al-Razzaq narrated to us, 'Ubayd Allah ibn 'Umar narrated to 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 major ritual impurity (junub)?" He said: "Yes, and he performs his ablution (wudu) for prayer." Nafi' said: So when Ibn 'Umar wanted to do any of that, he would perform his ablution for prayer, except for his feet.