Musnad Ahmad 898:3

'Affan narrated to us, Hammad narrated to us, from Humayd, from al-Hasan, from al-Muhajir ibn Qunfudh: "The Prophet, peace and blessings of Allah be upon him, was urinating, or had urinated, so I greeted him, but he did not return the greeting to me until he had performed ablution, then he returned the greeting to me."