Musnad Ahmad 790:119

Mu'ammal narrated to us, Hammad narrated to us, Humayd informed us, from al-Hasan, from 'Imran ibn Husayn, that he said: "We performed Tamattu' with the Messenger of Allah, may Allah bless him and grant him peace, and the Messenger of Allah, may Allah bless him and grant him peace, did not forbid us from it after that, and no prohibition regarding it was sent down from Allah."