Sahih Al Bukhari 1: Chapter 4, Hadith 151

(151) Getting water carried by somebody else for purification (washing one's private parts)

حَدَّثَنَا سُلَيْمَانُ بْنُ حَرْبٍ، ‏‏‏‏‏‏قَالَ:‏‏‏‏ حَدَّثَنَا شُعْبَةُ، ‏‏‏‏‏‏عَنْ أَبِي مُعَاذٍ هُوَ عَطَاءُ بْنُ أَبِي مَيْمُونَةَ، ‏‏‏‏‏‏قَالَ:‏‏‏‏ سَمِعْتُ أَنَسًا، ‏‏‏‏‏‏يَقُولُ:‏‏‏‏ "كَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ إِذَا خَرَجَ لِحَاجَتِهِ تَبِعْتُهُ أَنَا وَغُلَامٌ مِنَّا مَعَنَا إِدَاوَةٌ مِنْ مَاءٍ".

SAHIH HADITH:

Narrated Anas: Whenever Allah's Apostle went to answer the call of nature, I along with another boy from us used to go behind him with a tumbler full of water.

REFERENCE:

Sahih Al Bukhari 1: Chapter 4, Hadith 151