Sahih Al Bukhari 1: Chapter 4, Hadith 150

(150) To wash the private parts with water after answering the call of nature

حَدَّثَنَا أَبُو الْوَلِيدِ هِشَامُ بْنُ عَبْدِ الْمَلِكِ، ‏‏‏‏‏‏قَالَ:‏‏‏‏ حَدَّثَنَا شُعْبَةُ، ‏‏‏‏‏‏عَنْ أَبِي مُعَاذٍ وَاسْمُهُ عَطَاءُ بْنُ أَبِي مَيْمُونَةَ، ‏‏‏‏‏‏قَالَ:‏‏‏‏ سَمِعْتُ أَنَسَ بْنَ مَالِكٍ، ‏‏‏‏‏‏يَقُولُ:‏‏‏‏ "كَانَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ إِذَا خَرَجَ لِحَاجَتِهِ أَجِيءُ أَنَا وَغُلَامٌ مَعَنَا إِدَاوَةٌ مِنْ مَاءٍ، ‏‏‏‏‏‏يَعْنِي يَسْتَنْجِي بِهِ".

SAHIH HADITH:

Narrated Anas bin Malik: Whenever Allah's Apostle went to answer the call of nature, I along with another boy used to accompany him with a tumbler full of water. (Hisham commented, So that he might wash his private parts with it.)

REFERENCE:

Sahih Al Bukhari 1: Chapter 4, Hadith 150