Language:
كتاب الوضوء
4
Ablutions (Wudu')
(10)
Chapter: Providing water at lavatories (for washing the private parts after answering the call of nature)
(10)
باب وَضْعِ الْمَاءِ عِنْدَ الْخَلاَءِ

Narrated Ibn `Abbas:

Once the Prophet (ﷺ) entered a lavatory and I placed water for his ablution. He asked, "Who placed it?" He was informed accordingly and so he said, "O Allah! Make him (Ibn `Abbas) a learned scholar in religion (Islam).

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مُحَمَّدٍ، قَالَ حَدَّثَنَا هَاشِمُ بْنُ الْقَاسِمِ، قَالَ حَدَّثَنَا وَرْقَاءُ، عَنْ عُبَيْدِ اللَّهِ بْنِ أَبِي يَزِيدَ، عَنِ ابْنِ عَبَّاسٍ، أَنَّ النَّبِيَّ صلى الله عليه وسلم دَخَلَ الْخَلاَءَ، فَوَضَعْتُ لَهُ وَضُوءًا قَالَ ‏"‏ مَنْ وَضَعَ هَذَا ‏"‏‏.‏ فَأُخْبِرَ فَقَالَ ‏"‏ اللَّهُمَّ فَقِّهْهُ فِي الدِّينِ ‏"‏‏.‏
Reference : Sahih al-Bukhari 143
In-book reference : Book 4, Hadith 9
USC-MSA web (English) reference : Vol. 1, Book 4, Hadith 145
  (deprecated numbering scheme)