If Allah casts the desire to engage a woman in the heart of a man, it is Okay for him to look at her
Authentic hadithعن محمد بن مسلمة، قال: خطبت امرأة، فجعلتُ أتَخَبّأُ لها، حتى نظرتُ إليها في نَخْلٍ لها، فقيل له: أتفعلُ هذا وأنت صاحب رسول الله صلى الله عليه وسلم ؟ فقال: سمعت رسول الله صلى الله عليه وسلم يقول: «إذا ألقى الله في قلب امرئ خِطبة امرأة، فلا بأس أن ينظر إليها».
Muhammad ibn Maslamah (may Allah be pleased with him) reported: I engaged a woman, so I would hide for her among palm trees that belonged to her until I saw her. It was said to him: "Did you do that despite the fact that you are a Companion of the Messenger of Allah (may Allah's peace and blessings be upon him)?" He said: "I heard the Messenger of Allah (may Allah's peace and blessings be upon him) say: 'If Allah casts the desire to engage a woman in the heart of a man, it is Okay for him to look at her.'"
Explanation
This Hadīth shows that Muhammad ibn Maslamah (may Allah be pleased with him) wanted to engage a woman. He would hide for her so as to see her. A Tābi‘i saw him and was amazed at his attitude. So Muhammad told him that he based his behavior on the Prophet's command that if a man wants to engage a woman, such that Allah has cast an inclination to marrying her in his heart, he should look at her. Therefore, it is recommended to look at one's prospective wife even without her knowledge. In this respect, one might hide for her, which is only permissible for this necessity.
Source · Narrated by Ibn Majah - Narrated by Ahmad
From the Hadith Encyclopedia (hadeethenc.com).