Sunan an-Nasai Book 1, Hadith 18

أَخْبَرَنَا إِسْحَاقُ بْنُ إِبْرَاهِيمَ، قَالَ أَنْبَأَنَا عِيسَى بْنُ يُونُسَ، قَالَ أَنْبَأَنَا الأَعْمَشُ، عَنْ شَقِيقٍ، عَنْ حُذَيْفَةَ، قَالَ كُنْتُ أَمْشِي مَعَ رَسُولِ اللَّهِ صلى الله عليه وسلم فَانْتَهَى إِلَى سُبَاطَةِ قَوْمٍ فَبَالَ قَائِمًا فَتَنَحَّيْتُ عَنْهُ فَدَعَانِي وَكُنْتُ عِنْدَ عَقِبَيْهِ حَتَّى فَرَغَ ثُمَّ تَوَضَّأَ وَمَسَحَ عَلَى خُفَّيْهِ ‏.‏

It was narrated that Hudhaifah said:"I was walking with the Messenger of Allah (ﷺ) and he came to some people's garbage dump and urinated while standing up. I turned to go away, but he called me back (to conceal him), and I was just behind him. Then when he had finished, he performed Wudu' and wiped over his Khuffs