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. (DE)
English reference (DE): Vol. 1, Book 1, Hadith 350 (DE)
Arabic reference (DE): Book 1, Hadith 377 (DE)