Musnad Ahmad 898:1

Rawh narrated to us, Sa'id narrated to us, from Qatadah, from al-Hasan, from Hudayn Abu Sasan al-Raqashi, from al-Muhajir ibn Qunfudh ibn Umayr ibn Jud'an, who said: I greeted the Prophet, peace and blessings of Allah be upon him, while he was performing ablution, but he did not return the greeting to me. When he finished his ablution, he said: "Nothing prevented me from returning your greeting except that I was not in a state of ablution."