Sahih Al Bukhari – Book of Wedlock, Marriage (Nikaah)

Volumn 007, Book 062, Hadith Number 167.
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Narated By 'Abdullah bin Mas'ud : The Prophet said, "A woman should not look at or touch another woman to describe her to her husband in such a way as if he was actually looking at her." 

the reason is that the husband will start thinking about other women with his imagination which may be the start of an evil act in future… this Hadith protects wives 🙂