Musnad Ahmad 898:2

'Abd al-Wahhab narrated to us, from Sa'id, and Muhammad ibn Ja'far, Sa'id narrated to us, from Qatadah, from al-Hasan, from Hudayn Abu Sasan, from al-Muhajir ibn Qunfudh—'Abd al-Wahhab said: Ibn 'Umayr ibn Jud'an— That he greeted the Messenger of Allah (peace and blessings of Allah be upon him) while he was performing ablution, and he did not return the greeting to him. When he finished his ablution, he said: "Nothing prevented me from returning your greeting except that I disliked to mention Allah unless I was in a state of purity."