Sunan Ibn Majah, Chapter: The Chapters of Purification and its Sunnah

It was narrated that Muhajir bin Qunfudh bin (Umair) bin Judhan said: "I came to the Prophet (saw) 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 did not have ablution.'" (Daif) Another chain with similar wording.