"When any one of you wakes from sleep, let him not put his hand into the vessel until he has washed it three times, because he does not know where his hand spent the night." (Using translation from Nasāʾī 161)  

أَنَّ رَسُولَ اللهِ ﷺ قَالَ إِذَا اسْتَيْقَظَ أَحَدُكُمْ مِنْ مَنَامِهِ فَلَا يُدْخِلْ يَدَهُ فِي الْإِنَاءِ حَتَّى يُفْرِغَ عَلَيْهَا ثَلَاثَ مَرَّاتٍ فَإِنَّهُ لَا يَدْرِي أَيْنَ بَاتَتْ يَدَهُ