Sahih Muslim 278a
Sahih Muslim · Book 2: The Book of Purification · It is disliked for the person who wants to perform wudu’, and others, to put his hand in the vessel (containing water) before washing it three times, if he is not sure whether something impure is on his hands or not
Abu Huraira said:
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 " .
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.