It is narrated from Sayyiduna Ali (may Allah be pleased with him) that he said: The Messenger of Allah (peace and blessings be upon him) said to me: "O Ali! There is a treasure for you in Paradise, and you are the master of its two sides (or two ages). So do not follow one glance (at a non-mahram woman) with another, for the first glance (being unintentional) is forgiven for you, but you have no right to the second." The chain of this hadith is sahih (authentic), but the two Shaykhs did not record it.