Muwatta Malik 37

حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنْ أَبِي الزِّنَادِ، عَنِ الأَعْرَجِ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ إِذَا اسْتَيْقَظَ أَحَدُكُمْ مِنْ نَوْمِهِ فَلْيَغْسِلْ يَدَهُ قَبْلَ أَنْ يُدْخِلَهَا فِي وَضُوئِهِ فَإِنَّ أَحَدَكُمْ لاَ يَدْرِي أَيْنَ بَاتَتْ يَدُهُ ‏"‏ ‏.‏

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

Reference

Muwatta Malik 37

In-book

Book 2, Hadith 6