"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  

[Machine] Juda'an reported that he greeted the Messenger of Allah ﷺ while he was performing ablution but the Messenger of Allah ﷺ did not reply to him. When he finished his ablution, the Messenger of Allah ﷺ said, "I did not respond to you because I disliked mentioning Allah except in a state of purity."  

جُدْعَانَأَنَّهُ سَلَّمَ عَلَى رَسُولِ اللهِ ﷺ وَهُوَ يَتَوَضَّأُ فَلَمْ يَرُدَّ عَلَيْهِ فَلَمَّا فَرَغَ مِنْ وُضُوئِهِ قَالَ إِنَّهُ لَمْ يَمْنَعْنِي أَنْ أَرُدَّ عَلَيْكَ إِلَّا أَنِّي كَرِهْتُ أَنْ أَذْكُرَ اللهَ إِلَّا عَلَى طَهَارَةٍ  

[Machine] He greeted the Messenger of Allah ﷺ while he was performing ablution, but he did not respond to him. So, when he finished his ablution, he said, "The only reason I did not respond to you was because I disliked remembering Allah except in a state of purity."  

أَنَّهُ سَلَّمَ عَلَى رَسُولِ اللهِ ﷺ وَهُوَ يَتَوَضَّأُ فَلَمْ يَرُدَّ عَلَيْهِ فَلَمَّا فَرَغَ مِنْ وُضُوئِهِ قَالَ إِنَّهُ لَمْ يمْنَعْنِي أَنْ أَرُدَّ عَلَيْكَ إِلَّا أَنِّي كَرِهْتُ أَنْ أَذْكُرَ اللهَ إِلَّا عَلَى طَهَارَةٍ