Musnad Ahmad 749:0

Narrated to us by Muhammad ibn Ja’far, narrated to us by Shu’bah, from Qatadah, from al-Hasan, from al-Muhajir ibn Qunfudh, that he came to the Prophet (ﷺ) while he was performing ablution, so he greeted him, but he did not return the greeting to him until he had finished his ablution. Then he apologized to him and said: "Nothing prevented me from returning your greeting except that I disliked to mention the name of Allah except while I was in a state of purity."