Musnad Ahmad 31:126

Yahya ibn Ishaq narrated to us, Ibn Lahi'ah informed us, from Ubayd Allah ibn al-Mughirah, I heard Abu al-Haytham saying: I heard Abu Sa'id al-Khudri saying: "I saw the whiteness of the flank of the Messenger of Allah, may Allah bless him and grant him peace, while he was prostrating."