Maliks Muwatta Book 2, Hadith Number 9.
Section : The Wudu of a Man who has been Asleep when he Gets Up to Pray.
Yahya related to me from Malik from Abu’zZinad from al-Araj from Abu Hurayra that the Messenger of Allah, may Allah bless him and grant him peace, said, “When you wake up from sleep to pray, wash your hands before you put them in the wudu water, for you do not know where your hands have spent the night.”