"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