Language:
كتاب الوضوء
4
Ablutions (Wudu')
(35)
Chapter: (What is said regarding) a man who helps his companion to perform ablution (by pouring water for him)
(35)
باب الرَّجُلِ يُوَضِّئُ صَاحِبَهُ

Narrated Usama bin Zaid:

"When Allah's Messenger (ﷺ) departed from `Arafat, he turned towards a mountain pass where he answered the call of nature. (After he had finished) I poured water and he performed ablution and then I said to him, "O Allah's Messenger (ﷺ)! Will you offer the prayer?" He replied, "The Musalla (place of the prayer) is ahead of you (in Al-Muzdalifa).

حَدَّثَنِي مُحَمَّدُ بْنُ سَلاَمٍ، قَالَ أَخْبَرَنَا يَزِيدُ بْنُ هَارُونَ، عَنْ يَحْيَى، عَنْ مُوسَى بْنِ عُقْبَةَ، عَنْ كُرَيْبٍ، مَوْلَى ابْنِ عَبَّاسٍ عَنْ أُسَامَةَ بْنِ زَيْدٍ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم لَمَّا أَفَاضَ مِنْ عَرَفَةَ عَدَلَ إِلَى الشِّعْبِ، فَقَضَى حَاجَتَهُ‏.‏ قَالَ أُسَامَةُ بْنُ زَيْدٍ فَجَعَلْتُ أَصُبُّ عَلَيْهِ وَيَتَوَضَّأُ فَقُلْتُ يَا رَسُولَ اللَّهِ أَتُصَلِّي فَقَالَ ‏ "‏ الْمُصَلَّى أَمَامَكَ ‏"‏‏.‏
Reference : Sahih al-Bukhari 181
In-book reference : Book 4, Hadith 47
USC-MSA web (English) reference : Vol. 1, Book 4, Hadith 181
  (deprecated numbering scheme)