كتاب الطهارة
2
The Book of Purification
(26)
Chapter: 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
(26)
باب كَرَاهَةِ غَمْسِ الْمُتَوَضِّئِ وَغَيْرِهِ يَدَهُ الْمَشْكُوكَ فِي نَجَاسَتِهَا فِي الإِنَاءِ قَبْلَ غَسْلِهَا ثَلاَثًا
Sahih Muslim 278a

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.
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 ‏"‏ ‏.‏
Reference : Sahih Muslim 278a
In-book reference : Book 2, Hadith 109
USC-MSA web (English) reference : Book 2, Hadith 541
  (deprecated numbering scheme)