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. (Using translation from Nasāʾī 38)  

أَنَّهُ «سَلَّمَ عَلَى النَّبِيِّ ﷺ وَهُوَ يَبُولُ فَلَمْ يَرُدَّ عَلَيْهِ السَّلَامَ حَتَّى تَوَضَّأَ فَلَمَّا تَوَضَّأَ رَدَّهُ عَلَيْهِ»