(The narrator said:) I was sitting with Sayyiduna Muhammad bin Maslamah al-Ansari (may Allah be pleased with him) in his house when he saw a girl of the Ansar and followed her with his eyes. I said to him: May Allah have mercy on you, you look at her like this? He said: I heard the Messenger of Allah (peace and blessings be upon him) say: "When Allah puts into a person's heart (the thought of) proposing marriage to a woman, there is no harm in his looking at her." Abd al-Wahid bin Ziyad narrated it thus from al-Hajjaj, from Muhammad bin Sulayman bin Abi Hathmah, from his father.
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 16552