It is narrated from Sayyiduna Ali (may Allah be pleased with him) that once the Prophet (peace and blessings be upon him) said to him: "O Ali! A treasure has been kept for you in Paradise and you are the owner of its two horns (the complete owner), so if your gaze falls upon a non-mahram, do not look at her again, because the first glance will be forgiven for you but the second will not be forgiven."
Hadith Referenceمسند احمد / مسند الخلفاء الراشدين / 1373
Hadith Gradingحکم دارالسلام:حسن لغيره، وهذا إسناد ضعيف، محمد بن اسحاق مدلس وقد عنعن