Musnad Ahmad 31:873

Bahz narrated to us, Shu'bah narrated to us, and Hajjaj narrated to me, Shu'bah narrated to me, Qatadah informed us, from Abdullah ibn Abi Utbah—Hajjaj said: Ibn Utbah, the freed slave of Anas ibn Malik—he said: I heard Abu Sa'id al-Khudri saying: "The Messenger of Allah (peace and blessings of Allah be upon him) was more bashful than a virgin in her private chamber. And if he disliked something, we would recognize it in his face."