Musnad Ahmad 749:1

Muhammad ibn Ja‘far narrated to us, he said: It was asked about a man to whom a greeting is given while he is not in a state of ablution? He said: Sa‘id narrated to us, from Qatadah, from al-Hasan, from al-Hudayn Abu Sasan, from al-Muhajir ibn Qunfudh, that he greeted the Messenger of Allah (peace and blessings of Allah be upon him) while he was performing ablution, so he did not return the greeting to him until he had finished his ablution. Then he returned it to him and said: "Nothing prevented me from returning your greeting except that I disliked to mention Allah unless I was in a state of purity." He said: Because of this hadith, al-Hasan used to dislike reciting or mentioning Allah, the Mighty and Majestic, until he had purified himself.