The Book of Purification
وَحَدَّثَنَا نَصْرُ بْنُ عَلِيٍّ الْجَهْضَمِيُّ، وَحَامِدُ بْنُ عُمَرَ الْبَكْرَاوِيُّ، قَالاَ حَدَّثَنَا بِشْرُ بْنُ الْمُفَضَّلِ، عَنْ خَالِدٍ، عَنْ عَبْدِ اللَّهِ بْنِ شَقِيقٍ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ النَّبِيَّ صلى الله عليه وسلم قَالَ " إِذَا اسْتَيْقَظَ أَحَدُكُمْ مِنْ نَوْمِهِ فَلاَ يَغْمِسْ يَدَهُ فِي الإِنَاءِ حَتَّى يَغْسِلَهَا ثَلاَثًا فَإِنَّهُ لاَ يَدْرِي أَيْنَ بَاتَتْ يَدُهُ " .Abu Huraira said:When anyone amongst you wakes up from sleep, he must not put his hand in the utensil till he has washed it three times, for he does not know where his hand was during the night
From Sahih Muslim, whose reports the scholars accept as authentic.