"I came to the Prophet when he was performing ablution and greeted him with the Salam, but he did not return (the greeting). When he had finished his ablution he said: 'Nothing prevented me from returning your greeting but the fact that I need to have ablution.'" (Da'if) Another chain with similar wording.  

أَتَيْتُ النَّبِيَّ ﷺ وَهُوَ يَتَوَضَّأُ فَسَلَّمْتُ عَلَيْهِ فَلَمْ يَرُدَّ عَلَىَّ السَّلاَمَ فَلَمَّا فَرَغَ مِنْ وُضُوئِهِ قَالَ إِنَّهُ لَمْ يَمْنَعْنِي مِنْ أَنْ أَرُدَّ عَلَيْكَ إِلاَّ أَنِّي كُنْتُ عَلَى غَيْرِ وُضُوءٍ قَالَ أَبُو الْحَسَنِ بْنُ سَلَمَةَ حَدَّثَنَا أَبُو حَاتِمٍ حَدَّثَنَا الأَنْصَارِيُّ عَنْ سَعِيدِ بْنِ أَبِي عَرُوبَةَ فَذَكَرَ نَحْوَهُ  

"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."  

مَرَّ رَجُلٌ عَلَى النَّبِيِّ ﷺ وَهُوَ يَبُولُ فَسَلَّمَ عَلَيْهِ فَلَمْ يَرُدَّ عَلَيْهِ فَلَمَّا فَرَغَ ضَرَبَ بِكَفَّيْهِ الأَرْضَ فَتَيَمَّمَ ثُمَّ رَدَّ عَلَيْهِ السَّلاَمَ  

"A man passed by the Prophet while he was urinating, and greeted him by the Salam. The Messenger of Allah said to him: "If you see me in this situation, do not greet me with the Salam, for if you do that I will not respond to you.'"  

أَنَّ رَجُلاً مَرَّ عَلَى النَّبِيِّ ﷺ وَهُوَ يَبُولُ فَسَلَّمَ عَلَيْهِ فَقَالَ لَهُ رَسُولُ اللَّهِ ﷺ إِذَا رَأَيْتَنِي عَلَى مِثْلِ هَذِهِ الْحَالَةِ فَلاَ تُسَلِّمْ عَلَىَّ فَإِنَّكَ إِنْ فَعَلْتَ ذَلِكَ لَمْ أَرُدَّ عَلَيْكَ  

"A man passed by the Prophet while he was urinating and greeted him with the Salam, and he did not return the greeting."  

مَرَّ رَجُلٌ عَلَى النَّبِيِّ ﷺ وَهُوَ يَبُولُ فَسَلَّمَ عَلَيْهِ فَلَمْ يَرُدَّ عَلَيْهِ