كتاب الطهارة وسننها
Read the English translation first, with the original Arabic directly underneath.
It was narrated that Muhajir bin Qunfudh bin (Umair) bin Jud'an said:
"I came to the Prophet (ﷺ) 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.
إِنَّهُ لَمْ يَمْنَعْنِي مِنْ أَنْ أَرُدَّ عَلَيْكَ إِلاَّ أَنِّي كُنْتُ عَلَى غَيْرِ وُضُوءٍ