Musnad Ahmad 790:126

'Affan narrated to us, Hammad narrated to us, Humayd informed us, from al-Hasan, from 'Imran ibn Husayn, who said: "We performed Tamattu' during the time of the Messenger of Allah, may Allah bless him and grant him peace, and he did not forbid us from it, and no prohibition was revealed regarding it."