Hadith
حَدَّثَنَا هِشَامُ بْنُ عَمَّارٍ، حَدَّثَنَا مَسْلَمَةُ بْنُ عُلَىٍّ، حَدَّثَنَا الأَوْزَاعِيُّ، عَنْ يَحْيَى بْنِ أَبِي كَثِيرٍ، عَنْ أَبِي سَلَمَةَ، عَنْ أَبِي هُرَيْرَةَ، قَالَ مَرَّ رَجُلٌ عَلَى النَّبِيِّ ـ صلى الله عليه وسلم ـ وَهُوَ يَبُولُ فَسَلَّمَ عَلَيْهِ فَلَمْ يَرُدَّ عَلَيْهِ فَلَمَّا فَرَغَ ضَرَبَ بِكَفَّيْهِ الأَرْضَ فَتَيَمَّمَ ثُمَّ رَدَّ عَلَيْهِ السَّلاَمَ .
It was narrated that Abu Hurairah said:"A man passed by the Prophet while he was urinating, and greeted him with the Salam, but he did not return the greeting. While he finished, he struck the ground with his palms and did dry ablution (Tayammum), then he returned the greeting
Sahih · Al-Albani
Sunan Ibn Majah 351 ·Book 1, Hadith 85·Also available on sunnah.com·