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. (Using translation from Muslim 278a)   

أَنَّ النَّبِيَّ ﷺ قَالَ «إِذَا اسْتَيْقَظَ أَحَدُكُمْ مِنْ نَوْمِهِ فَلَا يَغْمِسْ يَدَهُ فِي وَضُوئِهِ حَتَّى يَغْسِلَهَا ثَلَاثًا فَإِنَّهُ لَا يَدْرِي حَيْثُ بَاتَتْ يَدُهُ»