Musnad Ahmad 31:692

Bahz narrated to us, Shu'bah narrated to us, Qatadah informed us, from Abdullah ibn Abi Utbah, who 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."