It is narrated from Sayyiduna Abu Umamah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "You will surely lower your gazes, and guard your private parts, and keep your faces upright, or else your faces will be darkened."
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 7809