Abu Huraira reported God's messenger as saying, “When one of you awakes from sleep he must not dip his hand in the vessel till he has washed it three times, for he does not know where his hand was during the night.” (Bukhari and Muslim.)

وَعَنْ أَبِي هُرَيْرَةَ رَضِيَ اللَّهُ عَنْهُ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «إِذَا اسْتَيْقَظَ أَحَدُكُمْ مِنْ نَوْمِهِ فَلَا يَغْمِسْ يَدَهُ فِي الْإِنَاءِ حَتَّى يَغْسِلَهَا فَإِنَّهُ لَا يَدْرِي أَيْنَ بَاتَتْ يَدُهُ»