Sunan an-Nasai Book 1, Hadith 38

أَخْبَرَنَا مُحَمَّدُ بْنُ بَشَّارٍ، قَالَ حَدَّثَنَا مُعَاذُ بْنُ مُعَاذٍ، قَالَ أَنْبَأَنَا سَعِيدٌ، عَنْ قَتَادَةَ، عَنِ الْحَسَنِ، عَنْ حُضَيْنٍ أَبِي سَاسَانَ، عَنِ الْمُهَاجِرِ بْنِ قُنْفُذٍ، أَنَّهُ سَلَّمَ عَلَى النَّبِيِّ صلى الله عليه وسلم وَهُوَ يَبُولُ فَلَمْ يَرُدَّ عَلَيْهِ السَّلاَمَ حَتَّى تَوَضَّأَ فَلَمَّا تَوَضَّأَ رَدَّ عَلَيْهِ ‏.‏

It was narrated from Al-Muhajir bin Qunfudh that he greeted the Prophet (ﷺ) with Salam while he was urinating, and he did not return the greeting until he had performed Wudu'. When he had performed Wudu' he returned the greeting