The Book of Purification
1/33. Chapter:
Greeting One Who Is Urinating
Sunan an-Nasai
أَخْبَرَنَا مَحْمُودُ بْنُ غَيْلاَنَ، حَدَّثَنَا زَيْدُ بْنُ الْحُبَابِ، وَقَبِيصَةُ، قَالاَ أَنْبَأَنَا سُفْيَانُ، عَنِ الضَّحَّاكِ بْنِ عُثْمَانَ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، قَالَ مَرَّ رَجُلٌ عَلَى النَّبِيِّ صلى الله عليه وسلم وَهُوَ يَبُولُ فَسَلَّمَ عَلَيْهِ فَلَمْ يَرُدَّ عَلَيْهِ السَّلاَمَ .
It was narrated that Ibn 'Umar said: "A man passed by the Prophet (ﷺ) when he was urinating and greeted him with Salam, but he did not return his greeting."
Reference : Sunan an-Nasa'i 37In-book reference : Book 1, Hadith 37English translation : Vol. 1, Book 1, Hadith 37