Sahih Bukhari Volume 5, Book 64, Hadith 4423

حَدَّثَنَا يَحْيَى بْنُ بُكَيْرٍ، عَنِ اللَّيْثِ، عَنْ عَبْدِ الْعَزِيزِ بْنِ أَبِي سَلَمَةَ، عَنْ سَعْدِ بْنِ إِبْرَاهِيمَ، عَنْ نَافِعِ بْنِ جُبَيْرٍ، عَنْ عُرْوَةَ بْنِ الْمُغِيرَةِ، عَنْ أَبِيهِ الْمُغِيرَةِ بْنِ شُعْبَةَ، قَالَ ذَهَبَ النَّبِيُّ صلى الله عليه وسلم لِبَعْضِ حَاجَتِهِ، فَقُمْتُ أَسْكُبُ عَلَيْهِ الْمَاءَ ـ لاَ أَعْلَمُهُ إِلاَّ قَالَ فِي غَزْوَةِ تَبُوكَ ـ فَغَسَلَ وَجْهَهُ، وَذَهَبَ يَغْسِلُ ذِرَاعَيْهِ فَضَاقَ عَلَيْهِ كُمُّ الْجُبَّةِ، فَأَخْرَجَهُمَا مِنْ تَحْتِ جُبَّتِهِ فَغَسَلَهُمَا ثُمَّ مَسَحَ عَلَى خُفَّيْهِ‏.‏

Narrated By Urwa bin Al-Mughira : Al-Mughira bin Shu'ba, said, "The Prophet went out to answer the call of nature and (when he had finished) I got up to pour water for him." I think that he said that the event had taken place during the Ghazwa of Tabuk. Al-Mughira added. "The Prophet washed his face, and when he wanted to wash his forearms, the sleeves of his cloak became tight over them, so he took them out from underneath the cloak and then he washed them (i.e. his forearms) and passed wet hands over his Khuffs."