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 278d

Abu Huraira reported:

The Apostle of Allah (ﷺ) said: When anyone amongst you wakes up from sleep, he should wash his hands three times before putting it in the utensil, for he does not know where his hand was during the night.
وَحَدَّثَنِي سَلَمَةُ بْنُ شَبِيبٍ، قَالَ حَدَّثَنَا الْحَسَنُ بْنُ أَعْيَنَ، حَدَّثَنَا مَعْقِلٌ، عَنْ أَبِي الزُّبَيْرِ، عَنْ جَابِرٍ، عَنْ أَبِي هُرَيْرَةَ، أَنَّهُ أَخْبَرَهُ أَنَّ النَّبِيَّ صلى الله عليه وسلم قَالَ ‏ "‏ إِذَا اسْتَيْقَظَ أَحَدُكُمْ فَلْيُفْرِغْ عَلَى يَدِهِ ثَلاَثَ مَرَّاتٍ قَبْلَ أَنْ يُدْخِلَ يَدَهُ فِي إِنَائِهِ فَإِنَّهُ لاَ يَدْرِي فِيمَ بَاتَتْ يَدُهُ ‏"‏ ‏.‏
Reference : Sahih Muslim 278d
In-book reference : Book 2, Hadith 112
USC-MSA web (English) reference : Book 2, Hadith 544
  (deprecated numbering scheme)