سنن ابن ماجه

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.