Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) said: I saw the whiteness of the side of the Prophet (peace and blessings be upon him) while he was in prostration.
Imam Ahmad recorded it; its chain contains Ibn Lahi'ah, about whom there is some criticism.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2750