My wife refuses to wear niqaab.

Answer

It is Waajib (compulsory) for women to cover their faces with a niqaab in the presence of males. This is a compulsory command of the Shariah.

Do not attempt to force your wife to wear it. She will rebel and the marriage may break down. Encourage her. Speak politely and affectionately to her about the need to wear Niqaab. Explain to her that even the dead body of a woman has to be wrapped in six shrouds. Then despite being so fully covered, a sheet is held over the grave when lowering her body to prevent strangers from looking at it.

Explain to her that Noor from the heavens settles on a woman who adopts rigid Hijaab (with niqaab) in the presence of strange (ghair mahram) males. Also mention that the woman whose face is exposed, thus enabling other men to stare at her, is cursed by Allah Ta'ala and His Angels. Therefore, Rasulullah (Sallallahu alayhi wasallam) said:

"Allah curses the one who looks (i.e. the man) and the one at whom he looks (i.e. the woman with the exposed face)."

A woman who attracts the gazes of men with her exposed face and even with her gaudy garments, is constantly under the Wrath of Allah Ta'ala. Tell your wife that when Rasulullah (Sallallahu alayhi wasallam) asked his beloved daughter, Hadhrat Faatimah (Radhiyallahu anha): "What is best for a woman?", she responded: "That no man should look at her nor should she look at any man." Delightfully, Rasulullah (Sallallahu alayhi w asallam) responded: "Faatimah is a portion of me."