Dr. `Abdul-Fattah `Ashoor, professor of Exegesis of the Qur’an at Al-Azhar University, states: “Islam prohibits spying and makes it a grave sin. Allah says: ‘O ye who believe! Shun much suspicion; for lo! Some suspicion is a crime. And spy not, neither backbite one another. Would one of you love to eat the flesh of his dead brother? Ye abhor that (so abhor the other)! And keep your duty (to Allah). Lo! Allah is Relenting, Merciful.’ (Al-Hujurat: 12)
Therefore, this act is not Islamic. The Companion who had hidden behind a tree intended to know if there were any physical defects in his would-be wife. Islam permits you to look at your would-be wife. You can know any other characters by means of intermingling with her family, visiting them frequently, and sitting with her in their presence. In this way, you will know her character.
Engagement is a period in which spouses are supposed to know each other. If they find themselves matching, then it will be okay, and marriage will come next. Stop that attitude or eavesdropping, and commit yourself to the Islamic morals, which will lead you to happiness.”