Sunan Ibn Majah 350

It was narrated that Muhajir bin Qunfudh bin (Umair) bin Jud'an said:"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