Sayyiduna Sahl bin Abi Hathmah al-Ansari (may Allah be pleased with him) said: I was sitting with Sayyiduna Muhammad bin Maslamah (may Allah be pleased with him) when the daughter of al-Dahhak passed behind him, and he began following her with his eyes. I said: Subhan Allah! You do this when you are a Companion of the Messenger of Allah (peace and blessings be upon him)? He said: I heard the Messenger of Allah (peace and blessings be upon him) say: "When Allah puts into a man's heart (the thought of) proposing marriage to a woman, there is no harm in his looking at her."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 16551