AL-SUNAN.org - Niqab-Ibn Taimiyyah Rahimahu Allah

Shaikh-ul-Islam Ibn Taymiyyah (Rahimahullah) relates: "Women used to room about without Cloaks (Jilbaabs) and men used to see their faces and hands, but when the verse stating 'O Prophet! Tell your wives and your daughters and the women of the believers to draw their cloaks over themselves.' (Surah Al-Ahzaab,Verse #59)was revealed, then this was prohibited and women were ordered to wear the Jilbaab. Then Ibn Tayimiyyah goes on to say "The word Jilbaab means a sheet which Ibn Mas'ud (Radhiallaahu Ánhu) explained as a cloak covering the entire body including the head, face and hands. Therefore, it is not permissible for the women to reveal the face and hands in public.
(Ibn Taymiyyah's book on fatwaas Page# 110 Vol # 2 also in the book Hijaab Page # 15)